CN115298338B

CN115298338B - Steel wire

Info

Publication number: CN115298338B
Application number: CN202180015878.0A
Authority: CN
Inventors: 寺本真也; 根石豊; 青野通匡; 峰田晓; 铃木章一; 越智达朗
Original assignee: Nippon Steel and Sumitomo Metal Corp; Nippon Steel SG Wire Co Ltd
Current assignee: Nippon Steel Corp; Nippon Steel SG Wire Co Ltd
Priority date: 2020-02-21
Filing date: 2021-02-19
Publication date: 2024-04-02
Anticipated expiration: 2041-02-19
Also published as: MX2022010291A; SE545842C2; JPWO2021167069A1; KR20220143735A; CN115298338A; JP7321353B2; WO2021167069A1; SE2251065A1; DE112021001166T5; US20230085279A1

Abstract

The present invention provides a steel wire having excellent cold rolling workability and excellent fatigue limit in the case of manufacturing a spring. The steel wire according to the present embodiment has a chemical composition comprising, in mass%: c:0.50 to 0.80 percent of Si:1.20 to less than 2.50 percent of Mn:0.25 to 1.00 percent of P: less than 0.020%, S: less than 0.020%, cr: 0.40-1.90%, V:0.05 to 0.60 percent of N:0.0100% or less, the balance being Fe and impurities, wherein the steel wire has a number density of V-based precipitates having a maximum diameter of 2 to 10nm of 5000 to 80000 particles/. Mu.m ³ 。

Description

Steel wire

Technical Field

The present invention relates to a steel wire, and more particularly, to a steel wire which is a material of springs typified by damper springs and valve springs.

Background

In a vehicle or a general machine, a large number of springs are utilized. Among springs used in vehicles and general machines, a damper spring has an effect of absorbing shock or vibration from the outside. Shock absorber springs, for example, are used to transmit power of a vehicle to a torque converter of a transmission. In the case where the damper spring is used in a torque converter, the damper spring absorbs vibrations of an internal combustion engine (e.g., an engine) of the vehicle. Therefore, a high fatigue limit is required for the damper spring.

Among springs used in vehicles and general machines, a valve spring has a function of adjusting opening and closing of a valve in the vehicle and general machine. Valve springs are used for controlling the opening and closing of an air supply and exhaust valve of an internal combustion engine (engine) of a vehicle. To adjust the valve opening and closing, the valve spring repeats thousands of compressions within 1 minute. Therefore, as with the damper springs, a high fatigue limit is required for the valve springs. The valve spring, in particular, repeats thousands of compressions at 1 minute, with a compression frequency much higher than the shock absorber spring. Thus, the valve spring requires a higher fatigue limit than the shock absorber spring. Specifically, for a damper spring, it is necessary to use a spring having a spring weight of 10 ⁷ With a higher fatigue limit at the number of repetitions, in contrast, a valve spring of 10 is required ⁸ With a higher fatigue limit at the number of repetitions.

An example of a method for manufacturing a spring represented by a damper spring and a valve spring is as follows. The steel wire is subjected to quenching and tempering (quenching treatment and tempering treatment). And (3) performing cold rolling on the quenched and tempered steel wire to form a coil-shaped intermediate steel material. And (5) carrying out stress relief annealing treatment on the intermediate steel. After the stress relief annealing treatment, nitriding treatment is performed as needed. That is, nitriding may or may not be performed. After the stress relief annealing treatment or nitriding treatment, shot peening is performed as needed to impart compressive residual stress to the surface layer. The spring is manufactured through the above steps.

Recently, it has been demanded to further improve the fatigue limit of springs.

The technology related to improving the fatigue limit of the spring is disclosed in: JP-A-2-57637 (patent document 1), JP-A-2010-163689 (patent document 2), JP-A-2007-302950 (patent document 3) and JP-A-2006-183137 (patent document 4).

The steel wire for high fatigue limit spring disclosed in patent document 1 contains C in weight%: 0.3 to 1.3 percent of Si:0.8 to 2.5 percent of Mn:0.5 to 2.0 percent of Cr:0.5 to 2.0% of Mo as an optional element: 0.1 to 0.5 percent, V:0.05 to 0.5 percent of Ti: 0.002-0.05%, nb: 0.005-0.2%, B:0.0003 to 0.01 percent, cu:0.1 to 2.0 percent of Al:0.01 to 0.1 percent of N: 1 or more than 2 of 0.01-0.05%, the balance being Fe and unavoidable impurities, and the steel is produced by air-cooling or rapid cooling after holding at 250-500 ℃ for 3 seconds to 30 minutes after austenitizing treatment, the yield ratio being 0.85 or less. In this document, a steel wire for a high fatigue limit spring having the above-described configuration is proposed based on the insight that the fatigue limit of a spring depends on the yield strength of the spring, and that the higher the yield strength of the spring is, the higher the fatigue limit of the spring is (see page 2, upper right column, line 1 to line 5 of patent document 1).

The spring disclosed in patent document 2 is manufactured using an oil tempered steel wire having a tempered martensite structure. The oil tempered steel wire contains, in mass%, C:0.50 to 0.75 percent, si:1.50 to 2.50 percent,mn:0.20 to 1.00 percent, cr: 0.70-2.20%, V:0.05 to 0.50 percent, and the balance of Fe and unavoidable impurities. When the oil-tempered steel wire is subjected to gas soft nitriding treatment at 450 ℃ for 2 hours, the lattice constant of the nitrided layer formed on the steel wire surface portion of the oil-tempered steel wire isWhen the oil-tempered steel wire is heated at 450 ℃ for 2 hours, the tensile strength is 1974MPa or more, the yield stress is 1769MPa or more, and the drawing value exceeds 40%. This document defines an oil tempered steel wire as a raw material for springs manufactured by nitriding treatment. In the case of manufacturing a spring by nitriding, as the nitriding time becomes longer, the yield strength and tensile strength of the steel material of the spring decrease. In this case, the hardness of the steel material is reduced and the fatigue limit is lowered. Accordingly, patent document 2 describes: by using an oil tempered steel wire in which the yield strength of the steel material does not decrease even if the nitriding treatment time is prolonged, a spring having a high fatigue limit can be manufactured (see paragraphs 0025 and 0026 of patent document 2).

The steel wire for high-strength spring disclosed in patent document 3 has the following chemical composition, and contains C:0.5 to 0.7 percent, si:1.5 to 2.5 percent, mn:0.2 to 1.0 percent, cr:1.0 to 3.0 percent, V:0.05 to 0.5 percent, and the inhibition is Al: less than 0.005% (excluding 0%), the remainder being Fe and unavoidable impurities. In the steel wire, the number of the spherical cementite bodies with the equivalent circular diameter of 10-100 nm is 30/mu m ² The Cr concentration in cementite is 20% or more by mass% and the V concentration is 2% or more. This document describes: the steel wire is effective in increasing the strength thereof for the improvement of the fatigue limit and the resistance to the reduction of the elastic force (see paragraph 0003 of patent document 3). And the number of fine spherical cementite bodies with equivalent circular diameter of 10-100 nm is set to 30/mu m ² As described above, the Cr concentration in cementite is 20% or more by mass%, and the V concentration is 2% or more, and the composition can be used even in a heat treatment such as a stress relief annealing treatment or a nitriding treatment in the production processCan suppress decomposition and disappearance of cementite and can maintain the strength of the steel wire (see paragraph [0011 ] of patent document 3])。

The steel wire as a material of the spring disclosed in patent document 4 includes, in mass%: c:0.45 to 0.7 percent, si:1.0 to 3.0 percent, mn:0.1 to 2.0 percent, P: less than 0.015%, S: less than 0.015%, N:0.0005 to 0.007%, t-O:0.0002 to 0.01%, and the balance of iron and unavoidable impurities, the tensile strength being 2000MPa or more, the occupied area ratio of cementite-based spherical carbide and alloy-based carbide in the mirror surface having an equivalent circle diameter of 0.2 μm or more being 7% or less, the existence density of cementite-based spherical carbide and alloy-based carbide having an equivalent circle diameter of 0.2 to 3 μm being 1/μm ² The cementite-based spherical carbide and alloy-based carbide having equivalent circular diameters exceeding 3 μm were present at a density of 0.001 pieces/μm ² Hereinafter, the prior austenite grain size is No. 10 or more, the retained austenite is 15mass% or less, and the area ratio of the lean region where the cementite-based spherical carbide having an equivalent circle diameter of 2 μm or more is present at a small density is 3% or less. This document describes: further, in order to further improve spring performance such as fatigue and spring force reduction, further improvement in strength is required. The document also describes: by controlling the microstructure and the distribution of fine carbides of cementite type, the spring can be made stronger, and the spring performance such as fatigue and spring force reduction can be improved (see paragraphs 0009 and 0021 of patent document 4).

Prior art literature

Patent literature

Patent document 1: japanese patent laid-open No. 2-57637

Patent document 2: japanese patent application laid-open No. 2010-163689

Patent document 3: japanese patent laid-open No. 2007-302950

Patent document 4: japanese patent laid-open No. 2006-183137

Disclosure of Invention

Problems to be solved by the invention

The techniques described in patent documents 1 to 4 are all discussed below, and the strength (hardness) of steel material and springs, which are spring materials, is increased to improve spring characteristics such as fatigue limit and spring force reduction. However, the fatigue limit of the spring may also be increased in other ways.

In the process of manufacturing the spring, cold rolling is performed on the steel wire as the spring material as described above. Therefore, a steel wire serving as a spring material may be required to have excellent cold rolling workability.

The purpose of the present invention is to provide a steel wire that has excellent cold rolling workability and that exhibits excellent fatigue limit when produced into a spring.

Means for solving the problems

The chemical composition of the steel wire of the present disclosure contains, in mass%:

C：0.50～0.80％、

si:1.20 to less than 2.50 percent,

Mn：0.25～1.00％、

P: less than 0.020%,

S: less than 0.020%,

Cr：0.40～1.90％、

V：0.05～0.60％、

N: the content of the organic light-emitting diode is less than 0.0100 percent,

the balance is composed of Fe and impurities,

in the steel wire, the number density of V-series precipitates with the maximum diameter of 2-10 nm is 5000-80000 pieces/mu m ³ 。

ADVANTAGEOUS EFFECTS OF INVENTION

The steel wire of the present disclosure has excellent cold rolling workability, and exhibits excellent fatigue limit in the case of a spring manufactured using the steel wire as a raw material.

Drawings

Fig. 1A is an example of a TEM image of the (001) plane of ferrite of a thin film sample.

Fig. 1B is a schematic view of a TEM image of the (001) face of the ferrite of the film sample.

FIG. 2 is a graph showing the Ca sulfide number ratios Rca and 10 in a valve spring having the chemical composition of the present embodiment ⁸ A graph of the relationship of the fatigue limit (high cycle fatigue limit) at the number of repetitions.

Fig. 3 is a flowchart showing a process for manufacturing a steel wire according to the present embodiment.

Fig. 4 is a flowchart showing a process for manufacturing a spring using the steel wire according to the present embodiment.

Detailed Description

As described in patent documents 1 to 4, in the conventional spring technology, it is considered that the strength and hardness of steel materials constituting the spring have a positive correlation with the fatigue limit of the spring. Thus, it is common knowledge of spring technology that the strength and hardness of a spring (steel material constituting the spring) have a positive correlation with the fatigue limit of the spring. Therefore, conventionally, the fatigue limit of a spring is predicted based on the strength of a steel material obtained by a tensile test completed in a short time or the hardness of a steel material obtained by a hardness test completed in a short time, instead of a very time-consuming fatigue test. That is, the fatigue limit of the spring is predicted from the result of the tensile test or the hardness test, which does not take time, without performing the time-consuming fatigue test.

However, the inventors believe that the strength and hardness of the spring (steel material constituting the spring) are not necessarily related to the fatigue limit of the spring. Therefore, it is studied to improve the fatigue limit of the spring by other technical ideas, not by improving the strength and hardness of the spring.

Here, the present inventors focused on V-based precipitates typified by V carbides and V carbonitrides. The V-based precipitate in the present specification means a precipitate containing V, or containing V and Cr. The V-based precipitate may not contain Cr. The inventors of the present invention have conceived to increase the fatigue limit of a spring manufactured from a steel wire as a raw material by forming a large amount of fine V-system precipitates having a nano size in the steel wire.

In addition, excellent cold workability (cold workability) is sometimes required for steel wires that are spring materials. In order to improve cold rolling workability, it is effective to suppress the Si content. Therefore, the present inventors have studied a steel wire which can sufficiently use nano-sized V-system precipitates to improve the fatigue limit of a spring and which can obtain excellent cold rolling workability from the viewpoint of chemical composition for the first time. As a result, the inventors of the present invention considered that the chemical composition of the steel wire as the spring material contains, in mass%: c:0.50 to 0.80 percent of Si:1.20 to less than 2.50 percent of Mn:0.25 to 1.00 percent of P: less than 0.020%, S: less than 0.020%, cr: 0.40-1.90%, V:0.05 to 0.60 percent of N: less than 0.0100%, ca:0 to 0.0050 percent, mo:0 to 0.50 percent of Nb:0 to 0.050 percent, W:0 to 0.60 percent of Ni:0 to 0.500 percent of Co:0 to 0.30 percent, B:0 to 0.0050 percent, cu:0 to 0.050 percent, al:0 to 0.0050%, and Ti:0 to 0.050% and the balance of Fe and impurities. Then, steel materials having the above chemical composition are subjected to heat treatment at various heat treatment temperatures after quenching treatment to produce steel wires, and then springs are produced using the steel wires. Then, the fatigue limit of the spring and the fatigue limit ratio defined as the ratio of the fatigue limit to the hardness of the spring (i.e., the fatigue limit ratio=the fatigue limit/the hardness of the spring) were investigated.

As a result of the investigation, the present inventors have found the following new findings with respect to steel wires having the above chemical composition. As described in the background art above, in the case of manufacturing a spring, nitriding treatment may be performed or nitriding treatment may not be performed. In the case of nitriding in the conventional spring manufacturing process, the heat treatment is performed at a temperature lower than the nitriding temperature of the nitriding in the heat treatment (e.g., the stress relief annealing process) after the quenching and tempering process. This is because the conventional spring manufacturing process is based on the technical idea of improving the fatigue limit of the spring by keeping the strength and hardness of the spring high. When nitriding is performed, heating at a nitriding temperature or lower is required. Therefore, in the conventional manufacturing process, the heat treatment temperature in the heat treatment process other than nitriding is set to be as lower than the nitriding temperature as possible, and the strength of the spring is suppressed from being lowered.

However, in the steel wire according to the present embodiment, a large amount of fine V-system precipitates having a nano size are formed to improve the fatigue strength of the springA limited technical idea is not a technical idea of increasing the fatigue limit of the spring by increasing the strength of the spring. Thus, by the study of the present inventors, it was found that: in the production process, if a large amount of fine V-system precipitates of nano-size are precipitated by heat treatment at a heat treatment temperature of 540 to 650 ℃, even if the heat treatment temperature for precipitating V-system precipitates is higher than the nitriding temperature, as a result, excellent fatigue limit can be obtained despite the decrease in the strength of the core portion of the spring (i.e., the low core hardness of the spring), and the fatigue limit ratio defined by the ratio of the fatigue limit to the core hardness of the spring can be improved. More specifically, through the study of the present inventors, it was found for the first time that: in a steel wire as a material of a spring, the number density of V-based precipitates having a maximum diameter of 2 to 10nm is 5000 pieces/μm ³ In the above manner, the spring manufactured by using the steel wire can obtain a sufficient fatigue limit.

As described above, the steel wire according to the present embodiment is obtained by a completely different technical idea from the prior art, and has the following structure.

[1] A steel wire comprising, in mass%, the following chemical components:

C：0.50～0.80％、

si:1.20 to less than 2.50 percent,

Mn：0.25～1.00％、

P: less than 0.020%,

S: less than 0.020%,

Cr：0.40～1.90％、

V：0.05～0.60％、

N: the content of the organic light-emitting diode is less than 0.0100 percent,

the balance is composed of Fe and impurities,

Here, as described above, the V-based precipitate is a carbide or carbonitride containing V or a carbide or carbonitride containing V and Cr, and is, for example, one or more of V carbide and V carbonitride. The V-based precipitate may be a composite precipitate containing one of V carbide and V carbonitride and 1 or more other elements. The V-system precipitate is precipitated in a plate shape along the {001} plane of ferrite (body-centered cubic lattice). Therefore, the V-system precipitate is observed as a line segment (edge portion) extending in a straight line parallel to the [100] direction or the [010] direction in the TEM image of the (001) plane of ferrite. Further, other precipitates than the V-based precipitates are not observed as line segments (edge portions) extending in a straight line parallel to the [100] direction or the [010] direction. That is, only V-based precipitates are observed as a line segment (edge portion) extending in a straight line parallel to the [100] direction or the [010] direction. Therefore, by observing a TEM image of the (001) plane of ferrite, V-based precipitates can be easily distinguished from Fe carbides such as cementite, and V-based precipitates can be identified. That is, in the present specification, in the TEM image of the (001) plane of ferrite, a line segment extending in the [100] direction or the [010] direction is defined as a V-based precipitate.

[2] The steel wire according to item [1], wherein,

the chemical composition contains Ca: at most 0.0050% by weight,

among the inclusions of the material of the steel, the material of the steel is,

an oxide inclusion is defined as an inclusion having an O content of 10.0% or more by mass%,

inclusions having an S content of 10.0% or more and an O content of less than 10.0% by mass% are defined as sulfide-based inclusions,

among the sulfide-based inclusions, when an inclusion having a Ca content of 10.0% or more and an S content of 10.0% or more and an O content of less than 10.0% in mass% is defined as Ca sulfide,

the ratio of the number of Ca sulfides to the total number of oxide inclusions is 0.20% or less.

As mentioned above, the valve spring repeats thousands of compressions at 1 minute, which is much more frequent than the shock absorber spring. Thus, the valve spring requires a higher fatigue limit than the shock absorber spring. Specifically, in the case of a damper spring, a spring load of 10 is required ⁷ Repeating times of timesIn contrast to higher fatigue limits, in the case of valve springs, a valve spring of 10 is required ⁸ With a higher fatigue limit at the number of repetitions. Hereinafter, in this specification, 10 will be described ⁸ The fatigue limit at the number of repetitions is called the high cycle fatigue limit.

Among the inclusions, especially Ca sulfide affects the high cycle fatigue limit. As described above, among the inclusions, the inclusion having an O content of 10.0% or more by mass% is defined as an oxide-based inclusion. Inclusions having an S content of 10.0% or more and an O content of less than 10.0% by mass% are defined as sulfide-based inclusions. Among the sulfide-based inclusions, the inclusion having a Ca content of 10.0% or more, an S content of 10.0% or more, and an O content of less than 10.0% in mass% is defined as Ca sulfide. Ca sulfide is one of sulfide inclusions. In the valve spring, when the ratio of the number of oxide inclusions to the number of Ca sulfides in the sulfide inclusions is low, the valve spring is high in cycle (10 ⁸ Cycling) the fatigue limit is improved. More specifically, when the ratio of the number of Ca sulfides to the total number of oxide inclusions and sulfide inclusions is 0.20% or less, the high cycle fatigue limit is particularly improved.

The reason for this is considered as follows. In the valve spring, when the ratio of the number of Ca sulfides to the total number of oxide inclusions and sulfide inclusions is low, ca is sufficiently dissolved in the oxide inclusions and sulfide inclusions other than Ca sulfides. In this case, the oxide inclusions and the sulfide inclusions are sufficiently softened and miniaturized. Thus, it can be considered that: is less likely to cause cracks originating from oxide inclusions and sulfide inclusions, and is high in cycle (10 ⁸ Cycling) the fatigue limit is improved.

[3] The steel wire according to item [1] or [2], wherein,

the chemical composition contains 1 or more than 2 elements selected from the following elements:

mo: less than 0.50 percent,

Nb:0.050% or less,

W: less than 0.60 percent,

Ni: less than 0.500 percent,

Co:0.30% or less

B: less than 0.0050%.

[4] The steel wire according to any one of [1] to [3], wherein,

cu:0.050% or less,

Al:0.0050% or less

Ti:0.050% or less.

The steel wire according to the present embodiment will be described in detail below. The "%" concerning an element refers to% by mass unless otherwise specified.

[ chemical composition of Steel wire ]

The steel wire according to the present embodiment is used as a spring material. The chemical composition of the steel wire of the present embodiment contains the following elements.

C：0.50～0.80％

Carbon (C) increases the fatigue limit of springs manufactured from steel as a raw material. If the C content is less than 0.50%, the above-mentioned effects cannot be sufficiently obtained even if the other element content is within the range of the present embodiment. On the other hand, if the C content exceeds 0.80%, coarse cementite is generated. In this case, even if the content of other elements is within the range of the present embodiment, the ductility of the steel material that becomes the spring material is reduced. In addition, the fatigue limit of springs manufactured from this steel material is rather reduced. Therefore, the C content is 0.50 to 0.80%. The preferable lower limit of the C content is 0.51%, more preferably 0.52%, still more preferably 0.54%, still more preferably 0.56%. The preferable upper limit of the C content is 0.79%, more preferably 0.78%, still more preferably 0.76%, still more preferably 0.74%, still more preferably 0.72%, still more preferably 0.70%.

Si:1.20 to less than 2.50 percent

Silicon (Si) improves the fatigue limit of springs manufactured from steel materials, and also improves the spring force resistance of the springs. Si also deoxidizes the steel. Si also increases the temper softening resistance of the steel. Therefore, even after the heat treatment is performed in the process of manufacturing the spring, the strength of the spring can be maintained at a high level. If the Si content is less than 1.20%, the above-described effects cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the Si content is 2.50% or more, the strength of the steel material to be a spring material becomes high and the cold workability of the steel material is lowered even if the content of other elements is within the range of the present embodiment. Therefore, the Si content is 1.20 to less than 2.50%. The preferable lower limit of the Si content is 1.25%, more preferably 1.30%, more preferably 1.40%, more preferably 1.50%, more preferably 1.60%, more preferably 1.70%, more preferably 1.80%. The preferable upper limit of the Si content is 2.48%, more preferably 2.46%, still more preferably 2.45%, still more preferably 2.43%, still more preferably 2.40%.

Mn：0.25～1.00％

Manganese (Mn) improves the hardenability of the steel and increases the fatigue limit of the spring. If the Mn content is less than 0.25%, the above-described effects cannot be sufficiently obtained even if the other element content is within the range of the present embodiment. On the other hand, if the Mn content exceeds 1.00%, the strength of the steel material to be a spring material becomes high and the cold workability of the steel material is lowered even if the content of other elements falls within the range of the present embodiment. Therefore, the Mn content is 0.25 to 1.00%. The preferable lower limit of the Mn content is 0.27%, more preferably 0.29%, more preferably 0.35%, more preferably 0.40%, more preferably 0.50%, more preferably 0.55%. The preferable upper limit of the Mn content is 0.98%, more preferably 0.96%, still more preferably 0.90%, still more preferably 0.85%, still more preferably 0.80%.

P: less than 0.020%

Phosphorus (P) is an impurity. P segregates at grain boundaries, reducing the fatigue limit of the spring. Therefore, the P content is 0.020% or less. The preferable upper limit of the P content is 0.018%, more preferably 0.016%, still more preferably 0.014%, still more preferably 0.012%. The P content is preferably as low as possible. However, excessive reduction in the P content leads to an increase in manufacturing costs. Therefore, if the usual industrial production is considered, the lower limit of the P content is preferably more than 0%, more preferably 0.001%, and even more preferably 0.002%.

S: less than 0.020%

Sulfur (S) is an impurity. S segregates at grain boundaries in the same manner as P, and combines with Mn to form MnS, thereby reducing the fatigue limit of the spring. Therefore, the S content is 0.020% or less. The preferable upper limit of the S content is 0.018%, more preferably 0.016%, still more preferably 0.014%, still more preferably 0.012%. The S content is preferably as low as possible. However, excessive reduction in the S content leads to an increase in manufacturing cost. Therefore, if the usual industrial production is considered, the lower limit of the S content is preferably more than 0%, more preferably 0.001%, still more preferably 0.002%.

Cr：0.40～1.90％

Chromium (Cr) improves the hardenability of the steel and increases the fatigue limit of the spring. If the Cr content is less than 0.40%, the above-mentioned effects cannot be sufficiently obtained even if the other element content is within the range of the present embodiment. On the other hand, if the Cr content exceeds 1.90%, coarse Cr carbide is excessively generated and the fatigue limit of the spring is lowered even if the other element content falls within the range of the present embodiment. Therefore, the Cr content is 0.40 to 1.90%. The preferable lower limit of the Cr content is 0.42%, more preferably 0.45%, more preferably 0.50%, more preferably 0.60%, more preferably 0.80%, more preferably 1.00%, more preferably 1.20%. The preferable upper limit of the Cr content is 1.88%, more preferably 1.85%, still more preferably 1.80%, still more preferably 1.70%, still more preferably 1.60%.

V：0.05～0.60％

Vanadium (V) combines with C and/or N to form fine V-based precipitates, which increase the fatigue limit of the spring. If the V content is less than 0.05%, the above-described effects cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the V content exceeds 0.60%, the V-based precipitates coarsen and a large amount of V-based precipitates having a maximum diameter exceeding 10nm are formed even if the content of other elements falls within the range of the present embodiment. In this case, the fatigue limit of the spring is rather lowered. Therefore, the V content is 0.05 to 0.60%. The preferable lower limit of the V content is 0.06%, more preferably 0.07%, still more preferably 0.10%, still more preferably 0.15%, still more preferably 0.20%. The preferable upper limit of the V content is 0.59%, more preferably 0.58%, more preferably 0.55%, more preferably 0.50%, more preferably 0.45%, more preferably 0.40%.

N:0.0100% or less

Nitrogen (N) is an impurity. N combines with Al and Ti to form AlN and TiN, so that the fatigue limit of the spring is reduced. Therefore, the N content is 0.0100% or less. The preferable upper limit of the N content is 0.0090%, more preferably 0.0080%, still more preferably 0.0060%, still more preferably 0.0050%. The N content is preferably as low as possible. However, excessive reduction in the N content leads to an increase in manufacturing cost. Therefore, the lower limit of the N content is preferably more than 0%, more preferably 0.0001%, still more preferably 0.0005%

The balance of the chemical composition of the steel wire according to the present embodiment is composed of Fe and impurities. Here, the impurities are substances mixed from ores, scraps, manufacturing environments, and the like as raw materials in the industrial production of the steel wire, and are allowed within a range that does not adversely affect the steel wire of the present embodiment.

[ optional element (optional elements) ]

The chemical composition of the steel wire according to the present embodiment may further contain Ca instead of a part of Fe.

Ca: less than 0.0050%

Calcium (Ca) is an optional element, and may not be contained. That is, the Ca content may be 0%. When the content of Ca exceeds 0%, ca is contained in the oxide inclusions and sulfide inclusions, and these inclusions are softened. The softened oxide inclusions and sulfide inclusions are elongated and divided and thinned during hot rolling. Therefore, the fatigue limit of the spring becomes high, and particularly the high cycle fatigue limit becomes high. However, if the Ca content exceeds 0.0050%, coarse Ca sulfides and coarse oxide inclusions (Ca oxides) are formed, and the fatigue limit of the spring is lowered. Therefore, the Ca content is 0 to 0.0050%, and in the case of Ca, the Ca content is 0.0050% or less. The preferable lower limit of the Ca content is 0.0001%, more preferably 0.0002%, still more preferably 0.0003%, still more preferably 0.0004%, still more preferably 0.0005%. The preferable upper limit of the Ca content is 0.0048%, more preferably 0.0046%, more preferably 0.0040%, more preferably 0.0035%, more preferably 0.0025%, more preferably 0.0020%.

The steel wire according to the present embodiment may further contain 1 or 2 or more kinds selected from Mo, nb, W, ni, co and B instead of a part of Fe. These elements are optional elements, and all improve the fatigue limit of springs manufactured from steel wires as raw materials.

Mo: less than 0.50%

Molybdenum (Mo) is an optional element, and may not be contained. That is, the Mo content may be 0%. When the content of Mo exceeds 0%, mo improves the hardenability of the steel material and increases the fatigue limit of the spring. Mo also increases the temper softening resistance of the steel. Therefore, even after the heat treatment is performed in the process of manufacturing the spring, the strength of the spring can be maintained at a high level. If Mo is contained in a small amount, the above-described effects are obtained to some extent. However, if the Mo content exceeds 0.50%, the strength of the steel material to be a spring material becomes high and the cold workability of the steel material is lowered even if the content of other elements is within the range of the present embodiment. Therefore, the Mo content is 0 to 0.50%, and in the case of Mo, the Mo content is 0.50% or less. The lower limit of the Mo content is preferably more than 0%, more preferably 0.01%, still more preferably 0.05%, still more preferably 0.10%. The upper limit of the Mo content is preferably 0.45%, more preferably 0.40%, still more preferably 0.35%, and still more preferably 0.30%.

Nb: less than 0.050%

Niobium (Nb) is an optional element, and may be absent. That is, the Nb content may be 0%. In the case of containing Nb, that is, in the case where the Nb content exceeds 0%, nb combines with C and/or N to form carbide, nitride, or carbonitride (hereinafter, referred to as Nb carbonitride or the like). Nb carbonitride and the like make austenite grains finer, and improve the fatigue limit of the spring. If Nb is contained in a small amount, the above-mentioned effects are obtained to some extent. However, if the Nb content exceeds 0.050%, coarse Nb carbonitrides and the like are generated, and the fatigue limit of the spring is lowered. Therefore, the Nb content is 0 to 0.050%, and in the case of Nb, the Nb content is 0.050% or less. The lower limit of the Nb content is preferably more than 0%, more preferably 0.001%, still more preferably 0.005%, still more preferably 0.010%. The preferable upper limit of the Nb content is 0.048%, more preferably 0.046%, more preferably 0.042%, more preferably 0.038%, more preferably 0.035%, more preferably 0.030%, more preferably 0.025%.

W: less than 0.60 percent

Tungsten (W) is an optional element, and may be absent. That is, the W content may be 0%. When the content of W exceeds 0%, W improves the hardenability of the steel material and increases the fatigue limit of the spring. W also increases the temper softening resistance of the steel. Therefore, even after the heat treatment is performed in the process of manufacturing the spring, the strength of the spring can be maintained at a high level. If W is contained in a small amount, the above-mentioned effects are obtained to some extent. However, if the W content exceeds 0.60%, the strength of the steel material to be a spring material becomes high even if the content of other elements falls within the range of the present embodiment, and the cold workability of the steel material is lowered. Therefore, the W content is 0 to 0.60%, and in the case of containing W, the W content is 0.60% or less. The lower limit of the W content is preferably more than 0%, more preferably 0.01%, still more preferably 0.05%, and still more preferably 0.10%. The preferable upper limit of the W content is 0.55%, more preferably 0.50%, more preferably 0.45%, more preferably 0.40%, more preferably 0.35%, more preferably 0.30%.

Ni: less than 0.500%

Nickel (Ni) is an optional element, and may not be contained. That is, the Ni content may be 0%. When the Ni content exceeds 0%, ni improves the hardenability of the steel material and improves the fatigue limit of the spring. If Ni is contained in a small amount, the above-mentioned effects are obtained to some extent. However, if the Ni content exceeds 0.500%, the strength of the steel material to be a spring material becomes high and the cold workability of the steel material is lowered even if the content of other elements is within the range of the present embodiment. Therefore, the Ni content is 0 to 0.500%, and in the case of Ni, the Ni content is 0.500% or less. The lower limit of the Ni content is preferably more than 0%, more preferably 0.001%, more preferably 0.005%, more preferably 0.010%, more preferably 0.050%, more preferably 0.100%, more preferably 0.150%. The preferable upper limit of the Ni content is 0.450%, more preferably 0.400%, still more preferably 0.350%, still more preferably 0.300%, still more preferably 0.250%.

Co: less than 0.30%

Cobalt (Co) is an optional element, and may not be contained. That is, the Co content may be 0%. When the Co content exceeds 0%, co increases the temper softening resistance of the steel. Therefore, even after the heat treatment is performed in the process of manufacturing the spring, the strength of the spring can be maintained at a high level. If Co is contained in a small amount, the above-mentioned effects are obtained to some extent. However, if the Co content exceeds 0.30%, the strength of the steel material to be a spring material becomes high and the cold workability of the steel material is lowered even if the content of other elements falls within the range of the present embodiment. Therefore, the Co content is 0 to 0.30%, and in the case of Co, the Co content is 0.30% or less. The lower limit of the Co content is preferably more than 0%, more preferably 0.01%, even more preferably 0.05%, and even more preferably 0.10%. The upper limit of the Co content is preferably 0.28%, more preferably 0.26%, and even more preferably 0.24%.

B: less than 0.0050%

Boron (B) is an optional element, and may be absent. That is, the B content may be 0%. When the content of B exceeds 0%, B improves the hardenability of the steel material and increases the fatigue limit of the spring. If B is contained in a small amount, the above-mentioned effects are obtained to some extent. However, if the B content exceeds 0.0050%, the strength of the steel material to be a spring material becomes high and the cold workability of the steel material is lowered even if the other element content falls within the range of the present embodiment. Accordingly, the content of B is 0 to 0.0050%, and when B is contained, the content of B is 0.0050% or less. The lower limit of the B content is preferably more than 0%, more preferably 0.0001%, further preferably 0.0010%, further preferably 0.0015%, further preferably 0.0020%. The preferable upper limit of the B content is 0.0049%, more preferably 0.0048%, further preferably 0.0046%, further preferably 0.0044%, further preferably 0.0042%.

The steel wire according to the present embodiment may further comprise a chemical composition selected from Cu: less than 0.050%, al:0.0050% or less, and Ti: 1 or 2 or more of 0.050% or less are used as impurities to replace part of Fe. If the content of these elements is within the above-mentioned range, the effects of the steel wire of the present embodiment and the spring manufactured using the steel wire can be obtained.

Cu: less than 0.050%

Copper (Cu) is an impurity, and may not be contained. That is, the Cu content may be 0%. Cu reduces the cold workability of the steel. If the Cu content exceeds 0.050%, the cold workability of the steel is significantly reduced even if the content of other elements is within the range of the present embodiment. Therefore, the Cu content is 0.050% or less. The Cu content may be 0%, and thus the Cu content is 0 to 0.050%. The upper limit of the Cu content is preferably 0.045%, more preferably 0.040%, more preferably 0.030%, more preferably 0.025%, more preferably 0.020%, more preferably 0.018%. As described above, the Cu content is preferably as low as possible. However, excessive reduction in Cu content leads to an increase in manufacturing cost. Therefore, the lower limit of the Cu content is preferably more than 0%, more preferably 0.001%, still more preferably 0.002%, still more preferably 0.005%.

Al: less than 0.0050%

Aluminum (Al) is an impurity, and may not be contained. That is, the Al content may be 0%. Al forms coarse oxide inclusions, which reduces the fatigue limit of the spring. If the Al content exceeds 0.0050%, the fatigue limit of the spring is significantly reduced even if the content of other elements is within the range of the present embodiment. Therefore, the Al content is 0.0050% or less. The Al content may be 0%, and thus the Al content is 0 to 0.0050%. The preferable upper limit of the Al content is 0.0045%, more preferably 0.0040%, further preferably 0.0030%, further preferably 0.0025%, further preferably 0.0020%. As described above, the Al content is preferably as low as possible. However, excessive reduction in Al content leads to an increase in manufacturing cost. Therefore, the lower limit of the Al content is preferably more than 0%, more preferably 0.0001%, further preferably 0.0003%, further preferably 0.0005%.

Ti: less than 0.050%

Titanium (Ti) is an impurity, and may not be contained. That is, the Ti content may be 0%. Ti forms coarse TiN. TiN tends to become the starting point for failure, reducing the fatigue limit of the spring. If the Ti content exceeds 0.050%, the fatigue limit of the spring is significantly reduced even if the other element content is within the range of the present embodiment. Therefore, the Ti content is 0.050% or less. The Ti content may be 0% and thus the Ti content is 0 to 0.050%. The preferable upper limit of the Ti content is 0.045%, more preferably 0.040%, still more preferably 0.030%, still more preferably 0.020%. As described above, the Ti content is preferably as low as possible. However, excessive reduction in the Ti content leads to an increase in manufacturing costs. Therefore, the lower limit of the Ti content is preferably more than 0%, more preferably 0.001%.

[ microstructure of Steel wire ]

The microstructure of the steel wire according to the present embodiment is a microstructure of a martensitic structure. Here, the term "microstructure is a microstructure of a martensite main body" means that the area ratio of martensite in the microstructure is 90.0% or more. In the present specification, martensite refers to tempered martensite. In the microstructure of the steel wire, phases other than martensite are precipitates, inclusions, and retained austenite. In these phases, precipitates and inclusions are small enough to be ignored compared with other phases.

The area ratio of martensite can be obtained by the following method. The test piece was collected by cutting in a direction perpendicular to the longitudinal direction of the steel wire according to the present embodiment. Among the surfaces of the collected test pieces, a surface corresponding to a cross section perpendicular to the longitudinal direction of the steel wire was set as an observation surface. After mirror polishing the observation surface, the observation surface was etched using 2% nitroethanol (nitroethanol etching solution). In the etched viewing surface, the central position of the line segment (i.e., radius R) from the surface of the steel wire to the center is defined as the R/2 position. The R/2 position of the observation surface was observed with a 500-fold optical microscope, and arbitrary 5-view photographic images were generated. The dimensions of each field of view were set to 100 μm×100 μm.

In each field of view, the contrast of each phase of martensite, retained austenite, precipitates, inclusions, and the like is different for each phase. Thus, based on the contrast, martensite is determined. The total area (. Mu.m) of martensite bodies defined in each field of view was determined ² ). The total area of martensite in the entire field of view was set to be 10000 μm relative to the total area of the entire field of view (10000 μm ² X 5) is defined as the area ratio (%) of martensite.

[ number Density of V-series precipitates in Steel wire ]

In the steel wire according to the present embodiment, the number density of V-based precipitates having a maximum diameter of 2 to 10nm is 5000 to 80000 pieces/μm ³ . In the present specification, the number density of V-based precipitates means the number density per unit volume (1. Mu.m in the present specification ³ ) The number of V-series precipitates.

In the present specification, the V-based precipitate is a precipitate containing V, or V and Cr. The V-based precipitates are, for example, V carbides and V carbonitrides. The V-based precipitate may be a composite precipitate containing one of V carbide and V carbonitride and other 1 or more elements. As described above, the V-based precipitate may not contain Cr. The V-system precipitate is precipitated in a plate shape along the {001} plane of ferrite. Therefore, in the TEM image of the (001) plane of ferrite, the V-system precipitate is observed as a line segment (edge portion) extending in a straight line parallel to the [100] direction or the [010] direction. Therefore, by observing a TEM image of the (001) plane of ferrite, V-based precipitates can be easily distinguished from Fe carbides such as cementite, and V-based precipitates can be identified.

In the steel wire manufactured by the manufacturing method described later, in which the content of each element in the chemical composition is within the range of the present embodiment, it was confirmed that the V-based precipitate was observed as a line segment (edge portion) extending in the [100] direction or the [010] direction in the TEM image of the (001) plane of ferrite, by analysis using an energy dispersive X-ray spectroscopy (Energy dispersiveX-ray diffraction: EDS) and a nanobeam diffraction pattern (Nano Beam Electron Diffraction: NBD).

Specifically, in the TEM image of the (001) plane of ferrite, V, or V and Cr can be detected if component analysis is performed by EDS on the precipitate observed as a line segment extending in the [100] direction or the [010] direction. If the crystal structure analysis by NBD is performed on the precipitate, the crystal structure of the precipitate is cubic, and the lattice constant is within a range of a=b=c=0.4167 nm±5%. In a database of the International diffraction data center (International Centerfor Diffraction Data:ICDD), the crystal structure of the V-system precipitate (V carbide and V carbonitride) was cubic, and the lattice constant was 0.4167nm (ICDD No. 065-8822).

In the steel wire according to the present embodiment, the fatigue limit of a spring manufactured using the steel wire can be improved by precipitating a large amount of nano-sized V-type precipitates having a maximum diameter of 2 to 10 nm. If the number density of V-based precipitates having a maximum diameter of 2 to 10nm is less than 5000 particles/μm ³ The V-group precipitates contributing to the improvement of the fatigue limit are too small. In this case, a sufficient fatigue limit for the spring cannot be obtained. The number density of V-based precipitates having a maximum diameter of 2 to 10nm is 5000 particles/μm ³ As described above, V-based precipitates are sufficiently present in the steel wire. Therefore, the fatigue limit of the spring is significantly increased. Maximum diameter of 2-10 nmThe preferable lower limit of the number density of V-series precipitates is 6000 pieces/μm ³ More preferably 7000 pieces/μm ³ More preferably 8000 pieces/μm ³ More preferably 10000 particles/μm ³ Further preferably 11000 pieces/μm ³ Further preferably 12000/μm ³ Further preferably 13000/μm ³ Further preferably 14000 pieces/μm ³ Further preferably 15000 pieces/μm ³ 。

The upper limit of the number density of V-based precipitates having a maximum diameter of 2 to 10nm is not particularly limited. However, in the case of the above chemical composition, the upper limit of the number density of V-based precipitates having a maximum diameter of 2 to 10nm is, for example, 80000 pieces/μm ³ . The upper limit of the number density of V-based precipitates having a maximum diameter of 2 to 10nm may be 75000/μm ³ 73000 units/μm may be used ³ 。

[ method for measuring number density of V-series precipitate ]

The number density of V-based precipitates having a maximum diameter of 2 to 10nm in the steel wire according to the present embodiment can be obtained by the following method. The steel wire according to the present embodiment was cut perpendicularly to the longitudinal direction thereof, and a disk having a surface (cross section) perpendicular to the longitudinal direction of the steel wire and a thickness of 0.5mm was collected. The disk was ground and polished from both sides using sandpaper, and the thickness of the disk was set to 50 μm. Then, a sample having a diameter of 3mm was collected from the disk. The sample was immersed in a 10% perchloric acid-glacial acetic acid solution, and electropolishing was performed to prepare a film sample having a thickness of 100 nm.

The film sample thus prepared was observed by a transmission electron microscope (Transmission Electron Microscope: TEM). Specifically, first, the chrysanthemum pool line is analyzed for the thin film sample, and the crystal orientation of the thin film sample is determined. Next, the thin film sample was set so that the (001) plane of ferrite (body-centered cubic lattice) could be observed by tilting the thin film sample based on the determined crystal orientation. Specifically, the film sample was inserted into a TEM and the chrysanthemum pool line was observed. The tilt of the thin film sample was adjusted so that the [001] direction of ferrite of the chrysanthemum pool line was aligned with the incidence direction of the electron beam. After adjustment, the real image is observed from the perpendicular direction of the (001) plane of ferrite. After setting, the field of view for the optional 4 observations of the film sample was determined. The observation magnification was 200000 times, the acceleration voltage was 200kV, and each observation field was observed. The field of view was set to 0.09 μm by 0.09. Mu.m.

Fig. 1A is an example of a TEM image of the (001) plane of ferrite of the thin film sample, and fig. 1B is a schematic view of a TEM image of the (001) plane of ferrite of the thin film sample. The axis indicated by [100] α in the drawing means the [100] direction in ferrite as the parent phase. The axis denoted by [010] α in the drawing means the [010] direction of ferrite as a parent phase. The V-system precipitate is precipitated in a plate shape along the {001} plane of ferrite. In ferrite grains of the (001) plane, V-based precipitates are observed as line segments (edge portions) extending linearly in the [100] direction or the [010] direction. In a TEM image, precipitates are represented by contrast with different brightness compared to the parent phase. Therefore, in the TEM image of the (001) plane of ferrite, a line segment extending in the [100] direction or the [010] direction is regarded as V-based precipitate. The length of the line segment of the V-based precipitate determined in the observation field was measured, and the length of the line segment obtained by the measurement was defined as the maximum diameter (nm) of the V-based precipitate. For example, symbol 10 (black line segment) in fig. 1A and 1B is a V-based precipitate.

The total number of V-based precipitates having a maximum diameter of 2 to 10nm in the 4-site observation field was obtained by the above measurement. Based on the total number of V-based precipitates obtained and the total area of the view field at 4 places, the number density (in/. Mu.m) of V-based precipitates having a maximum diameter of 2 to 10nm was obtained ³ )。

[ preferable Ca sulfide amount ratio Rca ]

In the present embodiment, oxide inclusions, sulfide inclusions, and Ca sulfide in the steel wire are defined as follows.

Oxide-based inclusions: inclusions having an O content of 10.0% or more in mass%

Sulfide-based inclusions: inclusions having an S content of 10.0% or more and an O content of less than 10.0% by mass

Ca sulfide: among the sulfide-based inclusions, inclusions having a Ca content of 10.0% or more, an S content of 10.0% or more, and an O content of less than 10.0% in mass percent are included

Oxide inclusions are selected from SiO, for example ₂ 、MnO、Al ₂ O ₃ And 1 or more than 2 kinds of MgO. The oxide inclusion may also be selected from SiO ₂ 、MnO、Al ₂ O ₃ Composite inclusions of 1 or more than 2 kinds of MgO and other alloy elements. The sulfide-based inclusion is, for example, 1 or more selected from MnS and CaS, and may be a composite inclusion containing 1 or more selected from MnS and CaS and other alloying elements. The Ca sulfide is, for example, caS, and may be a composite inclusion containing other alloying elements in CaS.

In the steel wire, the ratio of the number of Ca sulfides to the total number of oxide inclusions and sulfide inclusions is defined as Ca sulfide number ratio Rca (%). That is, rca is represented by the following formula.

Rca=number of Ca sulfides/number of oxide inclusions and total number of sulfide inclusions×100 (1)

In the present embodiment, preferably, ca is contained: 0.0050% or less, and the Ca sulfide number ratio Rca in the steel wire is 0.20% or less. Here, the Ca sulfide number ratio Rca in the steel wire means the Ca sulfide number ratio Rca at a position R/2 from the surface of the steel wire, in a case where R is a distance from the surface of the steel wire to the center axis (i.e., R is a radius of a section perpendicular to the longitudinal direction of the steel wire) (mm) in a section including the center axis of the steel wire (a section parallel to the longitudinal direction of the steel wire).

FIG. 2 shows the Ca sulfide number ratios Rca and 10 in a valve spring manufactured using a steel wire having the chemical composition of the present embodiment and having a Ca content of 0.0050% or less as a raw material ⁸ A graph of the relationship of the fatigue limit (high cycle fatigue limit) at the number of repetitions. Referring to fig. 2, when the Ca sulfide number ratio Rca exceeds 0.20%, the high cycle fatigue limit significantly increases as the Ca sulfide number ratio Rca becomes smaller. On the other hand, when the Ca sulfide number ratio Rca is 0.20% or less, Even if the Ca sulfide number ratio Rca is reduced, the high cycle fatigue limit does not become significantly large and is substantially constant. That is, in fig. 2, there is an inflection point in the vicinity of Ca sulfide number ratio rca=0.20%.

As described above, if the Ca sulfide number ratio Rca exceeds 0.20%, 10 ⁸ The fatigue limit (high cycle fatigue limit) at the number of repetitions decreases rapidly. If the Ca sulfide number ratio Rca is 0.20% or less, an excellent high cycle fatigue limit can be obtained. Therefore, in the steel wire according to the present embodiment, the Ca content is preferably more than 0 to 0.0050%, and the Ca sulfide number ratio Rca in the steel wire is preferably 0.20% or less. The upper limit of the Ca sulfide number ratio Rca is preferably 0.19%, more preferably 0.18%, and even more preferably 0.17%. The lower limit of the Ca sulfide number ratio Rca is not particularly limited, but in the case of the chemical composition described above, the lower limit of the Ca sulfide number ratio Rca is, for example, 0%, for example, 0.01%.

The Ca sulfide number ratio Rca was measured by the following method. Test pieces were collected from a cross section including the central axis of the steel wire according to the present embodiment. Among the surfaces of the collected test pieces, a surface corresponding to a cross section including the center axis of the steel wire was set as an observation surface. The observation surface is mirror polished. The observation field of view (each observation field of view: 100 μm×100 μm) at any 10 from the surface R/2 position of the steel wire in the mirror-polished observation plane was observed at a magnification of 1000 times using a Scanning Electron Microscope (SEM).

Inclusions in each observation field are determined based on the contrast in each observation field. For each inclusion identified, the EDS was used to identify oxide-based inclusions, sulfide-based inclusions, and Ca sulfide. Specifically, based on the elemental analysis result of EDS of the inclusions, the inclusion having an O content of 10.0% or more in mass% among the inclusions was determined as an "oxide-based inclusion". Among the inclusions, those having an S content of 10.0% or more and an O content of less than 10.0% by mass were identified as "sulfide-based inclusions". Among the specified sulfide-based inclusions, those having a Ca content of 10.0% or more, an S content of 10.0% or more, and an O content of less than 10.0% in mass% were specified as "Ca sulfide".

The inclusions to be identified are those having an equivalent circle diameter of 0.5 μm or more. Here, the equivalent circle diameter means a diameter of a circle in the case where the area of each inclusion is converted into a circle having the same area. When the equivalent circle diameter is 2 times or more the beam diameter of EDS, the accuracy of elemental analysis is improved. In the present embodiment, the beam diameter of EDS for specifying inclusions was set to 0.2 μm. In this case, in the case of inclusions having an equivalent circle diameter of less than 0.5 μm, the accuracy of the elemental analysis by EDS cannot be improved. Inclusions with an equivalent circle diameter below 0.5 μm have little effect on the fatigue limit of the spring. Therefore, in the present embodiment, inclusions having an equivalent circle diameter of 0.5 μm or more are targeted for determination. The upper limit of the equivalent circular diameter of the oxide inclusions, sulfide inclusions and Ca sulfide is not particularly limited, and is, for example, 100. Mu.m.

Based on the total number of oxide inclusions and sulfide inclusions determined in the observation field of view at 10 above and the total number of Ca sulfides determined in the observation field of view at 10 above, the Ca sulfide number ratio Rca (%) was determined using formula (1).

As described above, in the steel wire according to the present embodiment, the number density of V-based precipitates having a maximum diameter of 2 to 10nm is 5000 to 80000 pieces/μm within the range of the present embodiment as each element in chemical composition ³ . Therefore, the spring manufactured using the steel wire of the present embodiment has an excellent fatigue limit. Specifically, at 10 ⁷ With the number of repetitions, a high fatigue limit can be obtained. In this case, the steel wire according to the present embodiment is particularly suitable for damper spring applications.

The steel wire according to the present embodiment preferably further contains 0.0050% or less of Ca (i.e., the Ca content exceeds 0 to 0.0050%), and the Ca sulfide number ratio Rca is 0.20% or less. Therefore, the bullet manufactured by using the steel wire of the present embodimentThe spring can obtain a more excellent fatigue limit. Specifically, at 10 ⁸ With the number of repetitions, a high fatigue limit (high cycle fatigue limit) can be obtained. In this case, the steel wire of the present embodiment is particularly suitable for valve spring applications.

[ method for producing Steel wire ]

An example of the method for manufacturing a steel wire according to the present embodiment will be described below. The steel wire according to the present embodiment may have the above-described structure, and the manufacturing method is not limited to the following manufacturing method. Among them, the manufacturing method described below is a suitable example for manufacturing the steel wire of the present embodiment.

Fig. 3 is a flowchart showing an example of a process for manufacturing a steel wire according to the present embodiment. Referring to fig. 3, the method for manufacturing a steel wire according to the present embodiment includes a wire rod preparation step (S10) and a steel wire manufacturing step (S20). Hereinafter, each step will be described.

[ wire rod preparation step (S10) ]

The wire rod preparation step (S10) includes a raw material preparation step (S1) and a hot working step (S2). In the wire rod preparation step (S10), a wire rod serving as a raw material of the steel wire is manufactured.

[ raw Material preparation Process (S1) ]

In the raw material preparation step (S1), a raw material having the chemical composition described above is produced. The raw material referred to herein is a billet or a steel ingot. In the raw material preparation step (S1), first, molten steel having the above chemical composition is produced by a known refining method. Using the molten steel thus produced, a raw material (billet or ingot) is produced. Specifically, a billet is manufactured by a continuous casting method using molten steel. Alternatively, steel ingots are produced by ingot casting using molten steel. The subsequent hot rolling process is performed using a billet or ingot (S2).

[ Hot working Process (S2) ]

In the hot working step (S2), the raw material (billet or ingot) prepared in the raw material preparation step (S1) is subjected to hot rolling to produce a wire rod.

The hot rolling process step (S2) includes a rough rolling process step and a finish rolling process step. In the rough rolling step, first, the raw material is heated. Heating of the raw material uses a heating furnace or a soaking furnace. The raw materials are heated to 1200-1300 ℃ by a heating furnace or a soaking furnace. For example, the raw material is kept at a furnace temperature of 1200 to 1300 ℃ for 1.5 to 10.0 hours. And taking out the heated raw material from the heating furnace or the soaking furnace, and carrying out hot rolling. In the hot rolling in the rough rolling step, a cogging mill is used, for example. The raw material is bloomed by a blooming machine to manufacture a blank. When a continuous rolling mill is provided downstream of the cogging mill, hot rolling can be further performed on the cogged billet by using the continuous rolling mill, thereby further producing a billet having a small size. In a continuous rolling mill, for example, a horizontal stand having a pair of horizontal rolls and a vertical stand having a pair of vertical rolls are alternately arranged in a row. Through the above steps, in the rough rolling step, the raw material is manufactured into a billet.

In the finish rolling step, the billet after the rough rolling step is hot rolled to produce a wire rod. Specifically, the blank is charged into a heating furnace and heated at 900 to 1250 ℃. The heating time at a furnace temperature of 900 to 1250 ℃ is, for example, 0.5 to 5.0 hours. And taking out the heated blank from the heating furnace. The removed billet was hot rolled using a continuous rolling mill to produce a wire rod. The diameter of the wire rod is not particularly limited. The diameter of the wire may be determined based on the wire diameter of the spring as a final product. Through the above manufacturing steps, the wire rod is manufactured.

[ Steel wire production Process (S20) ]

In the steel wire manufacturing step (S20), the steel wire according to the present embodiment, which is a material of the spring, is manufactured. The steel wire is a steel material obtained by subjecting a wire rod as a hot-rolled material (hot-rolled material) to wire drawing processing 1 or more times. The steel wire manufacturing process (S20) includes: the patenting treatment step (S3), the drawing treatment step (S4), the tempering treatment step (S5), and the V-based precipitate formation heat treatment step (S100) are performed as needed.

[ Steel wire toughening treatment Process (S3) ]

In the patenting treatment step (S3), the wire rod produced in the wire rod preparation step (S10) is subjected to patenting treatment, and the microstructure of the wire rod is softened by assuming that the microstructure is a ferrite and pearlite structure. The patenting treatment may be performed by a known method. The heat treatment temperature in the patenting treatment is, for example, 550℃or higher, and more preferably 580℃or higher. The upper limit of the heat treatment temperature in patenting is 750 ℃. The patenting treatment step (S3) is not an essential step, but an optional step. That is, the patenting step (S3) may not be performed.

[ wire drawing Process (S4) ]

When the patenting step (S3) is performed, the wire rod after the patenting step (S3) is subjected to wire drawing in the wire drawing step (S4). When the patenting step (S3) is not performed, the wire rod after the hot rolling step (S2) is subjected to wire drawing in the wire drawing step (S4). By performing wire drawing, a steel wire having a desired diameter is manufactured. The drawing step (S4) may be performed by a known method. Specifically, the wire rod is subjected to a lubricating treatment, and a lubricating film typified by a phosphate film and a metal soap layer is formed on the surface of the wire rod. The wire rod after the lubrication treatment was subjected to wire drawing at room temperature. In the drawing process, a known drawing machine may be used. The wire drawing machine is provided with a die for wire drawing processing of the wire rod.

[ thermal refining Process (S5) ]

In the quenching and tempering step (S5), the steel wire after the wire drawing step (S4) is quenched and tempered. The quenching and tempering process (S5) includes a quenching process and a tempering process. In the quenching treatment step, first, the steel wire is heated to Ac ₃ Above the phase transition point. For heating, for example, a high-frequency induction heating device or a radiation heating device is used. The heated steel wire is rapidly cooled. The rapid cooling method may be water-cooled, or may be oil-cooled. The microstructure of the steel wire is a microstructure mainly composed of martensite through the quenching treatment step.

And (3) performing a tempering treatment process on the steel wire after the quenching treatment process. The tempering temperature in the tempering treatment procedure is Ac ₁ Below the phase transition point. The tempering temperature is, for example, 250 to 520 ℃. By performing the tempering treatment step, the microstructure of the steel wire is set to beThe structure of the tempered martensite body.

[ V-based precipitate formation Heat treatment Process (S100) ]

In the V-based precipitate formation heat treatment step (S100), the steel wire after the conditioning step (S5) is subjected to heat treatment (V-based precipitate formation heat treatment), and fine V-based precipitates are formed in the steel wire. By performing a V-based precipitate formation heat treatment step (S100), the number density of V-based precipitates having a maximum diameter of 2 to 10nm in a steel wire is set to 5000 to 80000 pieces/μm ³ 。

In the V-system precipitate formation heat treatment, the heat treatment temperature is set to 540 to 650 ℃. The holding time T (minutes) at the heat treatment temperature T (. Degree.C.) is not particularly limited, and is, for example, 5/60 (i.e., 5 seconds) to 50 minutes. The heat treatment temperature and the holding time are adjusted so that the number density of V-series precipitates with the maximum diameter of 2-10 nm in the steel wire is 5000-80000 pieces/mu m ³ 。

When the nitriding treatment step (S8) is performed in the spring manufacturing step described later, the heat treatment temperature in the V-group precipitate formation heat treatment may be higher than the nitriding temperature in the nitriding treatment step (S8). In the conventional spring manufacturing process, in the heat treatment (stress relief annealing process, etc.) after the tempering process, the heat treatment is performed at a temperature lower than the nitriding temperature in the case of performing the nitriding process (S8). This is because the conventional spring manufacturing process is based on a technical idea of improving the fatigue limit by keeping the strength and hardness of steel materials constituting the spring at high levels. When the nitriding treatment step (S8) is performed, heating at a nitriding temperature or lower is required. Therefore, in the conventional manufacturing process, the heat treatment temperature is set as low as possible in the heat treatment process other than the nitriding process, and the decrease in the strength of the spring (steel material constituting the spring) is suppressed. On the other hand, the steel wire according to the present embodiment adopts a technical idea of increasing the fatigue limit of the spring by forming a large amount of fine V-system precipitates having a nano size, rather than a technical idea of increasing the fatigue limit of the spring by increasing the strength of the spring (steel material constituting the spring). Therefore, in the V-based precipitate formation heat treatment, the heat treatment temperature is set to a temperature range of 540 to 650 ℃ at which V-based precipitates are likely to be formed. The lower limit of the heat treatment temperature in the V-system precipitate formation heat treatment is preferably 550 ℃, more preferably 560 ℃, further preferably 565 ℃, further preferably 570 ℃. The upper limit of the heat treatment temperature in the V-system precipitate formation heat treatment is preferably 640 ℃, more preferably 630 ℃, still more preferably 620 ℃, still more preferably 610 ℃.

In the V-based precipitate formation heat treatment, fn defined by the following formula (2) is further set to 29.5 to 38.9.

Fn＝{T ^3/2 ×{0.6t ^1/8 +(Cr+Mo+2V) ^1/2 }}/1000 (2)

T in the formula (2) is a heat treatment temperature (. Degree. C.) in the heat treatment for producing the V-based precipitate, and T is a holding time (minutes) at the heat treatment temperature T. The content (mass%) of the corresponding element in the chemical composition of the steel wire is substituted into each element symbol in the formula (2).

The amount of V-based precipitate is affected not only by the heat treatment temperature T (c) and the holding time T (min), but also by the contents of Cr, mo, and V, which are elements contributing to the formation of V-based precipitates.

Specifically, the formation of V-based precipitates is promoted by Cr and Mo. The reason for this is not clear, but the following reasons can be considered. Cr forms Fe-based carbide such as cementite or Cr carbide in a temperature range lower than the temperature range in which V-based precipitates are formed. Similarly, mo also forms Mo carbide (Mo ₂ C) A. The invention relates to a method for producing a fibre-reinforced plastic composite As the temperature increases, fe carbide, cr carbide, and Mo carbide form solid solutions, and become precipitation nuclei forming sites of V-based precipitates. As a result, the formation of V-based precipitates is promoted at the heat treatment temperature T.

If the content of each element in the chemical composition of the steel wire falls within the range of the present embodiment, the V-based precipitate formation heat treatment becomes insufficient when Fn is less than 29.5. In this case, the number density of V-based precipitates having a maximum diameter of 2 to 10nm in the steel wire producedBecomes less than 5000 pieces/μm ³ . On the other hand, if Fn exceeds 38.9, the V-system precipitate formed coarsens on the premise that the content of each element in the chemical composition of the steel wire falls within the range of the present embodiment. In this case, the number density of V-based precipitates having a maximum diameter of 2 to 10nm in the steel wire thus produced becomes less than 5000 particles/μm ³ 。

Under the condition that the content of each element in the chemical composition of the steel wire is within the range of the embodiment, when Fn is 29.5-38.9, the number density of V-series precipitates with the maximum diameter of 2-10 nm in the manufactured steel wire is 5000-80000/mu m ³ 。

The lower limit of Fn is preferably 29.6, more preferably 29.8, and still more preferably 30.0. The upper limit of Fn is preferably 38.5, more preferably 38.0, more preferably 37.5, more preferably 37.0, more preferably 36.5, more preferably 36.0, more preferably 35.5.

The steel wire according to the present embodiment can be manufactured through the above manufacturing steps. In the above-mentioned production process, the heat treatment process is carried out separately from the quenching and tempering process (S5) and the V-based precipitate formation heat treatment process (S100). However, the tempering step in the tempering step (S5) may be omitted, and the V-based precipitate formation heat treatment step (S100) may be performed after the quenching step. In this case, the steel wire after the quenching treatment step is subjected to a heat treatment (V-based precipitate formation heat treatment) at a heat treatment temperature T of 540 to 650 ℃ and an Fn of 29.5 to 38.9. In this way, the tempering step can be omitted, and the V-based precipitate formation heat treatment step can be performed after the quenching step. In this case, the V-based precipitate may be deposited and tempered simultaneously during the V-based precipitate formation heat treatment.

[ preferable production step for setting the Ca sulfide amount ratio Rca in the steel wire to 0.20% or less ]

The steel wire contains Ca: when the Ca sulfide number ratio Rca is 0.0050% or less and 0.20% or less, the raw material to be produced by performing the following refining step and casting step is preferably prepared in the raw material preparation step (S1).

[ refining Process ]

In the refining step, refining of molten steel and composition adjustment of molten steel are performed. The refining process includes primary refining and secondary refining. The primary refining is refining using a converter, and is known as refining. Secondary refining is refining using a ladle, and is known. In secondary refining, various alloy irons and auxiliary raw materials (slag formers) are added to molten steel. Typically, the alloyed iron and the secondary raw material contain Ca in various ways. Therefore, in order to control the Ca content and the Ca sulfide number ratio Rca in the valve spring manufactured by using the steel wire, it is important to (a) manage the Ca content contained in the alloyed iron and (B) add timing of the secondary raw material.

[ about (A) ]

Regarding (A) above, the Ca content in the alloyed iron is high. Therefore, in the case of the Si-deoxidized molten steel, the Ca yield in the molten steel is high. Therefore, in secondary refining, if alloy iron having a high Ca content is added, ca sulfide is excessively generated in the molten steel, so that the Ca sulfide number ratio Rca increases. Specifically, in secondary refining, when the Ca content in the alloy iron added to molten steel exceeds 1.0% by mass%, the Ca sulfide number ratio Rca exceeds 0.20%. Therefore, in secondary refining, the Ca content in the alloy iron added to the molten steel is set to 1.0% or less.

[ concerning (B) ]

In addition, with respect to the above (B), a secondary raw material (slag former) is added to the molten steel. The slag former is recycled slag containing quicklime, dolomite, ca oxide and the like. Ca added to the slag former of molten steel in the secondary refining in the refining step is contained in the slag former as Ca oxide. Thus, ca in the slag former enters the slag in secondary refining. However, when the slag former is added to the molten steel at the end of secondary refining, ca does not float up sufficiently, and remains in the molten steel without entering slag. In this case, the Ca sulfide number ratio Rca increases. Therefore, the slag former is added to the molten steel before the end of secondary refining. Here, "before the end of secondary refining" means that, when the refining time of secondary refining is defined as t (minutes), a time of at least 4t/5 minutes elapses from the start of secondary refining. That is, the slag former is added to the molten steel before 0.80t minutes have passed from the start of secondary refining in the refining process.

[ casting Process ]

The molten steel produced by the refining step is used to produce a raw material (billet or ingot). Specifically, a billet is manufactured by a continuous casting method using molten steel. Or steel ingots are produced by an ingot casting method using molten steel. Using the slab or ingot (raw material), the subsequent hot rolling process step (S2) is performed. The subsequent steps are as described above.

By performing the above manufacturing process, the following steel wire can be manufactured: the content of each element in the chemical composition is within the range of the present embodiment, ca is contained, the content of Ca is not more than 0.0050%, and the number density of V-series precipitates having a maximum diameter of 2-10 nm is 5000-80000 pieces/μm ³ The Ca sulfide number ratio Rca is 0.20% or less.

[ method for manufacturing spring Using Steel wire ]

Fig. 4 is a flowchart showing an example of a method for manufacturing a spring using the steel wire according to the present embodiment. The method for manufacturing a spring using a steel wire according to the present embodiment includes: a cold rolling step (S6), a stress relief annealing step (S7), a nitriding step (S8) which is performed as needed, and a shot peening step (S9).

[ Cold winding Process (S6) ]

In the cold rolling step (S6), the steel wire according to the present embodiment manufactured in the steel wire manufacturing step (S20) is cold rolled to manufacture an intermediate steel material for a spring. The cold rolls are manufactured using known coiling devices. The winding device includes, for example: a plurality of conveying roller groups, a wire guide, a plurality of coil forming jigs (winding pins), and a mandrel having a semicircular cross section. The conveying roller group includes a pair of rollers opposing each other. The plurality of conveying roller sets are arranged in a row. Each of the conveying roller groups sandwiches the steel wire between a pair of rollers and conveys the steel wire toward the wire guide. The steel wire passes through the wire guide. The steel wire from the wire guide is bent into an arc shape by a plurality of winding pins and a mandrel bar, and is formed into a coil-shaped intermediate steel material.

[ stress relief annealing Process (S7) ]

The stress relief annealing step (S7) is an essential step. In the stress relief annealing step (S7), annealing is performed to remove residual stress generated in the intermediate steel material in the cold rolling step (S6). The treatment temperature (annealing temperature) in the annealing treatment is set to, for example, 400 to 500 ℃. The holding time at the annealing temperature is not particularly limited, and is, for example, 10 to 50 minutes. After the holding time, the intermediate steel is cooled or slowly cooled to room temperature.

[ nitriding Process (S8) ]

The nitriding treatment step (S8) is an optional step, and is not necessarily a step. That is, the nitriding process may or may not be performed. In the nitriding treatment step (S8), the intermediate steel material after the stress relief annealing treatment step (S7) is subjected to nitriding treatment. In the nitriding treatment, nitrogen is introduced into the surface layer of the intermediate steel material, and a nitrided layer (hardened layer) is formed on the surface layer of the intermediate steel material by solid solution strengthening by solid solution nitrogen and precipitation strengthening by nitride formation.

The nitriding treatment may be performed under known conditions. In the nitriding treatment, at A _c1 The phase transition point or lower is performed at a treatment temperature (nitriding temperature). The nitriding temperature is, for example, 400 to 530 ℃. The holding time at the nitriding temperature is 1.0 to 5.0 hours. The atmosphere of the nitriding furnace is not particularly limited as long as it is sufficient to raise the chemical potential of nitrogen. As the nitriding atmosphere, for example, a gas atmosphere in which a carburizing gas (RX gas or the like) is mixed as in the soft nitriding treatment can be used.

[ shot peening step (S9) ]

The shot peening step (S9) is an essential step. In the shot peening step (S9), shot peening is performed on the surface of the intermediate steel material after the stress relief annealing step (S7) or the surface of the intermediate steel material after the nitriding step (S8). Thus, compressive residual stress can be applied to the surface layer of the spring, and the fatigue limit of the spring can be further improved. Shot peening may be performed by a known method. For shot peening, for example, a projection material having a diameter of 0.01 to 1.5mm is used. The projection material may be a known material such as steel shot or steel ball. The compressive residual stress applied to the spring is adjusted according to the diameter of the projection material, the projection speed, the projection time, and the projection amount per unit time per unit area.

The spring is manufactured through the above manufacturing steps. The springs are for example damper springs, valve springs. In the step of manufacturing the spring, the nitriding treatment step (S8) may be performed or not performed as described above. In other words, the nitriding treatment may be performed or not performed on the spring manufactured using the steel wire according to the present embodiment.

[ constitution of damper spring ]

In the case where the spring produced is a damper spring, the damper spring is coil-shaped. The wire diameter, coil average diameter, coil inner diameter, coil outer diameter, free height, effective number of windings, total number of windings, winding direction, and pitch of the damper spring are not particularly limited.

Among the damper springs, the damper spring subjected to nitriding treatment is referred to as a "nitrided damper spring". Among the damper springs, the damper spring without nitriding treatment is referred to as a "damper spring without nitriding treatment". The shock absorber spring subjected to nitriding treatment is provided with a nitriding layer and a core portion. The nitride layer includes a compound layer and a diffusion layer formed inside the compound layer. The nitride layer may not include a compound layer. The core portion is a base material portion located inside the nitriding layer, and is a portion that is substantially unaffected by nitrogen diffusion caused by the nitriding treatment. The nitrided layer and core in the nitrided damper spring can be distinguished by microscopic tissue observation. The shock absorber spring which is not nitrided does not have a nitrided layer.

When a nitrided damper spring is produced using the steel wire according to the present embodiment, the chemical composition of the core portion of the nitrided damper spring is the same as that of the steel wire according to the present embodiment, and the number density of V-based precipitates having a maximum diameter of 2 to 10nm is 5000 to 80000 pieces/μm ³ . Thus (2)The damper springs can achieve excellent fatigue limits. The microstructure of the core of the damper spring subjected to nitriding treatment is the same as that of the steel wire, and the area ratio of martensite is 90.0% or more.

In the case of producing a damper spring which is not nitrided by using the steel wire according to the present embodiment, the chemical composition is the same as that of the steel wire according to the present embodiment in the interior (arbitrary R/2 position (R is radius) of the cross section in the wire diameter direction) of the damper spring which is not nitrided, and the number density of V-based precipitates having a maximum diameter of 2 to 10nm at the R/2 position is 5000 to 80000 pieces/μm ³ . Therefore, even in the case of the damper spring which is not subjected to nitriding treatment, an excellent fatigue limit can be obtained. The microstructure at the R/2 position of the damper spring, which was not nitrided, was the same as that of the steel wire, and the area ratio of martensite was 90.0% or more.

[ constitution of valve spring ]

In the case where the spring produced is a valve spring, the valve spring is coil-shaped. The wire diameter, the coil average diameter, the coil inner diameter, the coil outer diameter, the free height, the effective number of windings, the total number of windings, the winding direction, and the pitch of the valve spring are not particularly limited.

Among the valve springs, the valve spring subjected to nitriding treatment is referred to as a "nitrided valve spring". Among the valve springs, the valve spring in which nitriding is omitted is referred to as a "valve spring which is not nitrided". The valve spring subjected to nitriding treatment is provided with a nitriding layer and a core portion. The nitride layer includes a compound layer and a diffusion layer formed inside the compound layer. The nitride layer may not include a compound layer. The core portion is a base material portion located inside the nitriding layer, and is a portion that is not substantially affected by diffusion of nitrogen generated by the nitriding treatment. The nitrided layer and core in the valve spring can be distinguished by microscopic tissue observation. The valve spring which is not nitrided is not provided with a nitrided layer.

In the case of manufacturing the valve spring subjected to nitriding treatment using the steel wire of the present embodiment, the valve spring subjected to nitriding treatmentThe chemical composition of the core part of the steel wire is the same as that of the steel wire of the present embodiment, and the number density of V-based precipitates having a maximum diameter of 2 to 10nm is 5000 to 80000 pieces/μm ³ . In the core, the Ca sulfide number ratio Rca is 0.20% or less. Thus, the nitrided valve spring can obtain an excellent high cycle fatigue limit. The microstructure of the core of the valve spring subjected to nitriding treatment is the same as that of the steel wire, and the area ratio of martensite is 90.0% or more.

When a valve spring which has not been nitrided is produced using the steel wire according to the present embodiment, the chemical composition is the same as that of the steel wire according to the present embodiment in the interior (optional R/2 position (R is radius) of the cross section in the wire diameter direction) of the valve spring which has not been nitrided, and the number density of V-based precipitates having a maximum diameter of 2 to 10nm at the R/2 position is 5000 to 80000 pieces/μm ³ . In addition, at the R/2 position, the Ca sulfide number ratio Rca is 0.20% or less. Therefore, even the valve spring which is not nitrided can obtain excellent high cycle fatigue limit. The microstructure at the R/2 position of the valve spring, which was not nitrided, was the same as that of the steel wire, and the area ratio of martensite was 90.0% or more.

The manufacturer of the steel wire according to the present embodiment can receive a wire rod supply from a third party and manufacture the steel wire using the prepared wire rod.

Example 1

The effects of the steel wire according to the present embodiment will be described in more detail by way of examples. The conditions in the following examples are one example of conditions used for confirming the possibility and effect of the steel wire according to the present embodiment. Therefore, the steel wire according to the present embodiment is not limited to this example of conditions.

[ production of Steel wire ]

In example 1, a steel wire as a material of a damper spring was manufactured. Then, the damper springs after nitriding treatment and those without nitriding treatment were manufactured using steel wires, and characteristics (fatigue limit) of the damper springs were examined. Specifically, molten steels having the chemical compositions of table 1 were produced.

The "-" portion in table 1 means that the corresponding element content is below the detection limit. That is, it means that the corresponding element is not included. For example, the Nb content of steel grade No. a is "0"% "when rounded to four decimal places. In the chemical compositions of steel grade numbers described in table 1, the balance other than the elements described in table 1 is Fe and impurities. Cast pieces (billets) were produced by continuous casting using the molten steel. After this billet was heated, cogging as a rough rolling step and subsequent rolling by a continuous rolling mill were performed, whereby a billet having a cross section of 162mm×162mm perpendicular to the longitudinal direction was produced. The heating temperature in the cogging is 1200-1250 ℃, and the holding time at the heating temperature is 2.0 hours.

Using the thus-produced billet, a finish rolling step was performed to produce a wire rod having a diameter of 5.5 mm. The heating temperature in the heating furnace of each test number in the finish rolling step was 1150 to 1200 ℃, and the holding time at the heating temperature was 1.5 hours.

The wire rod thus produced was subjected to patenting treatment. The heat treatment temperature in the patenting treatment is 650-700 ℃, and the holding time at the heat treatment temperature is 20 minutes. The wire rod after the patenting treatment was subjected to wire drawing to produce a steel wire having a diameter of 4.0 mm. The steel wire thus produced was subjected to a quenching treatment. The quenching temperature is 950-1000 ℃. The steel wire held at the quenching temperature is water-cooled. Tempering the quenched steel wire. The tempering temperature was 480 ℃. The steel wire after tempering is subjected to a V-system precipitate formation heat treatment. The heat treatment temperature T (c), the holding time T (min) at the heat treatment temperature T, and the Fn values in the V-system precipitate formation heat treatment are shown in table 2. In test numbers 24 and 25, no V-system precipitate formation heat treatment was performed. Steel wires of respective test numbers were manufactured through the above steps.

TABLE 2

[ manufacture of shock absorber spring ]

Using the produced steel wire, a damper spring subjected to nitriding treatment and a damper spring not subjected to nitriding treatment were produced. The damper spring subjected to nitriding treatment is manufactured by the following manufacturing method. The steel wire of each test number was cold rolled under the same conditions to produce a coiled intermediate steel material. The coil average diameter D of the coil-shaped intermediate steel material was 26.5mm, and the wire diameter D of the coil-shaped intermediate steel material was 4.0mm. The intermediate steel material is subjected to a stress relief annealing treatment. The annealing temperature in the stress relief annealing treatment was 450 ℃, and the holding time at the annealing temperature was 20 minutes. After the holding time has elapsed, the intermediate steel product is cooled. Nitriding the intermediate steel material after the stress relief annealing treatment. The nitriding temperature was set to 450℃and the holding time at the nitriding temperature was set to 5.0 hours. After nitriding, shot peening is performed under known conditions. First, shot peening was performed using a cutting line having a diameter of 0.8mm as a shot material. Next, shot peening was performed using steel shots having a diameter of 0.2mm as a casting material. The shot peening projection speed, projection time, and projection amount per unit time per unit area are the same in each test number. By the above manufacturing method, the damper spring subjected to nitriding treatment is manufactured.

The damper spring which has not been subjected to nitriding treatment is manufactured by the following manufacturing method. The steel wire of each test number was cold rolled under the same conditions to produce a coiled intermediate steel material. The intermediate steel material is subjected to a stress relief annealing treatment. The annealing temperature in the stress relief annealing treatment was 450 ℃, and the holding time at the annealing temperature was 20 minutes. After the holding time has elapsed, the intermediate steel product is cooled. After the stress relief annealing treatment, the nitriding treatment was not performed, and shot peening was performed under the same conditions as in the case of the damper springs subjected to the nitriding treatment. By the above manufacturing method, a damper spring that has not been subjected to nitriding treatment is manufactured. Through the above manufacturing steps, the damper springs (nitrided or non-nitrided) were manufactured.

[ evaluation test ]

For each of the produced steel wires of test numbers, cold rolling workability test, microstructure observation test, and V-series precipitate number density measurement test were performed. Further, microstructure observation tests, number density measurement tests of V-based precipitates, vickers hardness measurement tests, and fatigue tests were performed on each of the produced shock absorber springs (nitrided and non-nitrided).

[ Cold roll workability test ]

The steel wire of each test number was subjected to cold rolling under the following conditions, and whether cold rolling was performed was examined. The coil average diameter D of the coil-shaped intermediate steel material (= (coil inner diameter+coil outer diameter)/2) was set to 12.1mm, and the wire diameter D of the coil-shaped intermediate steel material was set to 4.0mm. The ability to perform cold rolling is shown in the column "ability to roll" in table 2. The case where cold rolling was possible was "o" and the case where cold rolling was not possible was "x".

[ microstructure observation test ]

The test pieces were collected by cutting the steel wire in a direction perpendicular to the longitudinal direction of the steel wire of each test number. Among the surfaces of the test pieces obtained by the collection, the surface corresponding to the cross section of the steel wire perpendicular to the longitudinal direction was regarded as the observation surface. After mirror polishing the observation surface, the observation surface was etched using 2% nitroethanol (nitroethanol etching solution). The R/2 position of the etched viewing surface was observed using a 500-fold optical microscope, yielding a photographic image of any 5 fields of view. The dimensions of each field of view were set to 100 μm×100 μm. The contrast of each phase such as martensite, retained austenite, precipitates, inclusions, and the like varies from phase to phase in each field of view. Thus, martensite is determined based on the contrast. The total area (. Mu.m) of martensite determined in each field of view was determined ² ). The total area of martensite in the entire field of view was set to be 10000 μm relative to the total area of the entire field of view (10000 μm ² X 5) is defined as the area fraction (%). The area ratios of the obtained martensite are shown in table 2. The nitrided damper springs of the respective test numbers were cut in the radial direction, and test pieces were collected. Further, each test number of the shock absorber spring which was not nitrided was cut along the linear direction, and test pieces were collected. For each of the collected test pieces, the above-mentioned microstructure observation test was performed. As a result, the area ratio of martensite in the core portion of the nitriding shock absorber spring of each test number and the area ratio of martensite in the non-nitriding shock absorber spring of each test number are the same as the area ratio of martensite in the steel wire of the corresponding test number.

[ number Density measurement test of V-series precipitate ]

The steel wire was cut perpendicularly to the longitudinal direction of each test number, and a disk having a surface (cross section) perpendicular to the longitudinal direction of the steel wire and a thickness of 0.5mm was collected. The disk was ground and polished from both sides using sandpaper to a thickness of 50 μm. Then, a sample having a diameter of 3mm was collected from the disk. The sample was immersed in a 10% perchloric acid-glacial acetic acid solution, and electropolishing was performed to prepare a film sample having a thickness of 100 nm.

The fabricated film samples were observed by TEM. Specifically, first, the thin film sample was analyzed for the chrysanthemum pool line, and the crystal orientation of the thin film sample was determined. Next, the thin film sample was set so that the (001) plane of ferrite (body-centered cubic lattice) could be observed by tilting the thin film sample based on the determined crystal orientation. Specifically, a thin film sample was inserted into the TEM and the chrysanthemum pool line was observed. The tilt of the thin film sample was adjusted so that the [001] direction of ferrite of the chrysanthemum pool line was identical to the incidence direction of the electron beam. After adjustment, when the real image is observed, the observation is made from the perpendicular direction of the (001) plane of ferrite. After setting, the field of view for any 4 of the film samples was determined. The observation magnification was 200000 times, the acceleration voltage was 200kV, and each observation field was observed. The field of view was set to 0.09 μm by 0.09. Mu.m.

As described above, V-based precipitates are precipitated in a plate shape along the {001} plane of ferrite. In ferrite grains of the (001) plane, V-based precipitates are observed as line segments (edge portions) extending linearly in the [100] direction or the [010] direction. In the TEM image, the precipitates are represented by contrast having different brightness from the parent phase. Therefore, in the TEM image of the (001) plane of ferrite, a line segment extending in the [100] direction or the [010] direction is regarded as V-based precipitate. The length of the line segment of the V-based precipitate determined in the observation field was measured, and the measured length of the line segment was defined as the maximum diameter (nm) of the V-based precipitate.

The total number of V-based precipitates having a maximum diameter of 2 to 10nm in the 4-site observation field was obtained by the above measurement. Based on the total number of V-based precipitates obtained and the total volume of the view field observed at 4 places, the number density (in/. Mu.m) of V-based precipitates having a maximum diameter of 2 to 10nm was obtained ³ ). The number density of V-based precipitates obtained was set to "number density of V-based precipitates (number/. Mu.m) in Table 2 ³ ) "A column shows. "number density of V-series precipitate (mu m) ³ ) "in a column" - "means that the number density of V-based precipitates is 0 pieces/μm ³ . The number density of V-system precipitates was also measured for each test number of the nitrided damper springs by the same method as that obtained in the steel wire. As a result, the number density of V-system precipitates in the core portion of the nitriding-treated damper spring of each test number is the same as the number density of V-system precipitates in the steel wire of the corresponding test number. The number density of V-system precipitates was also measured for each test number of damper springs that were not nitrided by the same method as that obtained in the steel wire. As a result, the number density of V-system precipitates of the damper springs without nitriding treatment for each test number is the same as the number density of V-system precipitates of the steel wire for the corresponding test number.

[ Vickers hardness measurement test ]

The hardness of the core of the nitrided damper spring of each test number was obtained by the vickers hardness measurement test. Specifically, a vickers hardness measurement test was performed in accordance with JIS Z2244 (2009) at any 3 of the R/2 positions of the cross section in the wire diameter direction of the nitrided damper spring of each test number. The test force was set at 0.49N. The arithmetic average of the obtained vickers hardness at 3 was taken as the vickers hardness of the core of the nitriding shock absorber spring of the test number.

Similarly, the hardness of the non-nitrided damper springs of each test number was obtained by the vickers hardness measurement test. Specifically, a vickers hardness measurement test based on JIS Z2244 (2009) was performed at any 3 of the R/2 positions of the cross section in the wire diameter direction of the shock absorber spring without nitriding treatment for each test number. The test force was set at 0.49N. The arithmetic average of the obtained vickers hardness at 3 was taken as the vickers hardness of the shock absorber spring without nitriding treatment of the test number.

[ fatigue test ]

The following fatigue tests were performed using the damper springs (nitrided and non-nitrided) of the respective test numbers. In the fatigue test, a compression fatigue test was performed in which a load was repeatedly applied to a coil-shaped damper spring (nitrided or non-nitrided) in the central axis direction. As the tester, an electrohydraulic servo type fatigue tester (load capacity 500 kN) was used.

The test conditions were: the load is applied at a stress ratio of 0.2, and the frequency is set to 1 to 3Hz. Repeating for 10 times ⁷ And secondly, the upper limit, is implemented until the shock absorber spring breaks. At up to 10 ⁷ If the secondary damper spring has not yet broken, the test is stopped and it is determined that the secondary damper spring has not broken. Here, will 10 ⁷ The maximum value of the test stress not broken at the time was set to F _M F is to F _M Above and reach 10 ⁷ The minimum value of the test stress at which the fracture occurred next time before was set to F _B . Will F _M And F _B Is set to F _A Will (F) _B -F _M )/F _A F in the case of.ltoreq.0.10 _A Defined as the fatigue limit (MPa). On the other hand, in the case where the test result is all broken, i.e., F cannot be obtained _M In the case of (2), extrapolation from the relationship between fracture life and test stress corresponds to 10 ⁷ Test stress for sub-life, the test stress obtained is defined as the fatigue limit (MPa). Here, test is performedThe test stress corresponds to the surface stress amplitude at the fracture site. For each test number of damper springs, the fatigue limit (MPa) was determined based on the above definition and evaluation test. Further, using the obtained fatigue limit and vickers hardness, the fatigue limit ratio (=fatigue limit/vickers hardness of the core) of the shock absorber spring subjected to nitriding treatment, and the fatigue limit ratio (=fatigue limit/vickers hardness) of the shock absorber spring not subjected to nitriding treatment were obtained.

[ test results ]

The test results are shown in Table 2. Referring to table 2, the chemical compositions of test numbers 1 to 21 were appropriate, and the manufacturing process was also appropriate. Therefore, the martensite area ratio was 90.0% or more in the microstructure of each test-numbered steel wire. In addition, the number density of V-series precipitates with the maximum diameter of 2-10 nm is 5000-80000/mu m ³ . Therefore, the fatigue limit of the nitrided damper spring manufactured from the steel wire as the raw material is 1470MPa or more, and the fatigue limit ratio (=fatigue limit/vickers hardness of core) of the nitrided damper spring is 2.55 or more. The fatigue limit of the shock absorber spring without nitriding produced using the steel wire is 1420MPa or more, and the fatigue limit ratio (=fatigue limit/vickers hardness) of the shock absorber spring without nitriding is 2.46 or more.

On the other hand, in test No. 22, the Si content was too high. Therefore, the workability of the cold roll is low.

In test No. 23, the V content was too low. Therefore, the number density of V-based precipitates at 2 to 10nm is too small in the steel wire. As a result, the fatigue limit of the shock absorber spring subjected to nitriding treatment was less than 1470MPa, and the fatigue limit ratio was less than 2.55. In addition, the fatigue limit of the shock absorber spring without nitriding treatment is lower than 1420MPa, and the fatigue limit ratio is lower than 2.46.

In test nos. 24 and 25, although the chemical composition was proper, the steel wire was not subjected to heat treatment for forming V-system precipitates. Therefore, the number density of V-based precipitates having a maximum diameter of 2 to 10nm is too small in the steel wire. As a result, the fatigue limit of the shock absorber spring subjected to nitriding treatment was less than 1470MPa, and the fatigue limit ratio was less than 2.55. In addition, the fatigue limit of the shock absorber spring without nitriding treatment is lower than 1420MPa, and the fatigue limit ratio is lower than 2.46.

In test nos. 26 to 28, the heat treatment temperature in the V-system precipitate formation heat treatment was too low, although the chemical composition was appropriate. Therefore, the number density of V-based precipitates having a maximum diameter of 2 to 10nm is too small in the steel wire. As a result, the fatigue limit of the shock absorber spring subjected to nitriding treatment was less than 1470MPa, and the fatigue limit ratio was less than 2.55. In addition, the fatigue limit of the shock absorber spring without nitriding treatment is lower than 1420MPa, and the fatigue limit ratio is lower than 2.46.

In test nos. 29 to 31, the heat treatment temperature in the V-system precipitate formation heat treatment was too high, although the chemical composition was appropriate. Therefore, in the steel wire, the V-based precipitates coarsen, and the number density of V-based precipitates having a maximum diameter of 2 to 10nm is too small. As a result, the fatigue limit of the shock absorber spring subjected to nitriding treatment was less than 1470MPa, and the fatigue limit ratio was less than 2.55. In addition, the fatigue limit of the shock absorber spring without nitriding treatment is lower than 1420MPa, and the fatigue limit ratio is lower than 2.46.

In test No. 32, although the chemical composition was appropriate, fn defined by the formula (2) exceeded 38.9 in the V-system precipitate formation heat treatment. As a result, the number density of V-based precipitates having a maximum diameter of 2 to 10nm in the steel wire is too small. As a result, the fatigue limit of the shock absorber spring subjected to nitriding treatment was less than 1470MPa, and the fatigue limit ratio was less than 2.55. In addition, the fatigue limit of the shock absorber spring without nitriding treatment is lower than 1420MPa, and the fatigue limit ratio is lower than 2.46.

In test No. 33, although the chemical composition was appropriate, fn defined by the formula (2) was lower than 29.5 in the V-system precipitate formation heat treatment. As a result, the number density of V-based precipitates having a maximum diameter of 2 to 10nm in the steel wire is too small. As a result, the fatigue limit of the shock absorber spring subjected to nitriding treatment was less than 1470MPa, and the fatigue limit ratio was less than 2.55. In addition, the fatigue limit of the shock absorber spring without nitriding treatment is lower than 1420MPa, and the fatigue limit ratio is lower than 2.46.

Example 2

[ production of Steel wire ]

In example 2, a steel wire serving as a material of a valve spring was manufactured. Then, the valve spring subjected to nitriding treatment and the valve spring not subjected to nitriding treatment were manufactured using steel wires, and the characteristics (fatigue limit) of the valve spring were examined. Specifically, molten steels having the chemical compositions of table 3 were produced.

The "-" portion in table 3 means that the corresponding element content is below the detection limit. In the chemical compositions of steel grade numbers shown in table 3, the balance other than the elements shown in table 3 is Fe and impurities. The refining conditions (Ca content (mass%) in the alloy iron added to the molten steel in the refining step and the time from the start of the refining step to the addition of the slag former when the refining time was set to t (minutes)) at the time of producing the molten steel are shown in table 4.

TABLE 4

The refined molten steel is used to manufacture a billet by continuous casting. After heating the billet, cogging as a rough rolling step and subsequent rolling by a continuous rolling mill were performed, and a billet having a cross section of 162mm×162mm perpendicular to the longitudinal direction was produced. The heating temperature in the cogging is 1200-1250 ℃, and the holding time at the heating temperature is 2.0 hours.

The obtained billet was subjected to a finish rolling step to produce a wire rod having a diameter of 5.5 mm. The heating temperature in the heating furnace of each test number in the finish rolling step was 1150 to 1200 ℃, and the holding time at the heating temperature was 1.5 hours.

The wire rod thus produced was subjected to patenting treatment. The heat treatment temperature in the patenting treatment is 650-700 ℃, and the holding time at the heat treatment temperature is 20 minutes. The wire rod after the patenting treatment was subjected to wire drawing to produce a steel wire having a diameter of 4.0 mm. The steel wire thus produced was subjected to a quenching treatment. The quenching temperature is 950-1000 ℃. The steel wire held at the quenching temperature is water-cooled. Tempering the quenched steel wire. The tempering temperature was 480 ℃. The steel wire after tempering is subjected to a V-system precipitate formation heat treatment. The heat treatment temperature T (. Degree.C.) during the heat treatment for producing the V-system precipitate, the holding time T (minutes) at the heat treatment temperature T, and the Fn values are shown in Table 4. The V-system precipitate formation heat treatment was not performed for test numbers 26 to 28. Through the above steps, steel wires of respective test numbers were manufactured.

[ manufacturing of valve spring ]

Nitriding treated valve springs and non-nitriding treated valve springs were manufactured using the manufactured steel wire. The valve spring subjected to nitriding treatment was manufactured by the following manufacturing method. The steel wire of each test number was cold rolled under the same conditions to produce a coiled intermediate steel material. The coil average diameter D of the coil-shaped intermediate steel material was 26.5mm, and the wire diameter D of the coil-shaped intermediate steel material was 4.0mm. The intermediate steel is subjected to a stress relief annealing treatment. The annealing temperature in the stress relief annealing treatment was 450 ℃, and the holding time at the annealing temperature was 20 minutes. After the holding time has elapsed, the intermediate steel product is cooled. Nitriding the intermediate steel material after the stress relief annealing treatment. The nitriding temperature was set to 450℃and the holding time at the nitriding temperature was set to 5.0 hours. After nitriding, shot peening is performed under known conditions. First, shot peening was performed using a cutting line having a diameter of 0.8mm as a shot material. Next, shot peening was performed using steel shots having a diameter of 0.2mm as a casting material. The projection speed, the projection time, and the projection amount per unit time per unit area in each shot peening are the same in each test number. The valve spring subjected to nitriding treatment was manufactured by the above manufacturing method.

The valve spring which has not been subjected to nitriding treatment is produced by the following production method. The steel wire of each test number was cold rolled under the same conditions to produce a coiled intermediate steel material. The intermediate steel material is subjected to a stress relief annealing treatment. The annealing temperature in the stress relief annealing treatment was 450 ℃, and the holding time at the annealing temperature was 20 minutes. After the holding time has elapsed, the intermediate steel product is cooled. After the stress relief annealing treatment, the nitriding treatment was not performed, and shot peening was performed under the same conditions as in the case of the valve spring subjected to the nitriding treatment. The valve spring which is not nitrided was manufactured by the above manufacturing method. The valve spring (nitrided or not) was produced by the above production steps.

[ evaluation test ]

The steel wire of each test number was subjected to cold rolling workability test, microstructure observation test, ca sulfide number ratio Rca measurement test, and V-system precipitate number density measurement test. Further, microstructure observation tests, number density measurement tests of V-based precipitates, vickers hardness measurement tests, and fatigue tests were performed on each of the valve springs (nitrided and non-nitrided) manufactured for each test number.

[ Cold roll workability test ]

The steel wire of each test number was subjected to cold rolling under the following conditions, and whether cold rolling was performed was examined. The coil average diameter D of the coil-shaped intermediate steel material (= (coil inner diameter+coil outer diameter)/2) was set to 12.1mm, and the wire diameter D of the coil-shaped intermediate steel material was set to 4.0mm. The ability to perform cold rolling is shown in the column "ability to roll" in table 4. The case where cold rolling was possible was "o" and the case where cold rolling was not possible was "x".

[ microstructure observation test ]

The martensite area ratio of each test-numbered steel wire was obtained by the same method as in the microstructure observation test in example 1. The area ratios of the obtained martensite are shown in table 4. The valve springs subjected to nitriding treatment of each test number were cut in the radial direction, and test pieces were collected. Further, each test number was cut along the linear direction to collect test pieces. The above-described microstructure observation test was performed on each of the collected test pieces. As a result, the area ratio of martensite in the core portion of the valve spring subjected to nitriding treatment for each test number and the area ratio of martensite in the valve spring subjected to non-nitriding treatment for each test number are the same as the area ratio of martensite in the steel wire for the corresponding test number.

[ number Density measurement test of V-series precipitate ]

The number density of V-group precipitates in each test number was obtained by the same method as the number density measurement test of V-group precipitates in example 1. Specifically, the steel wire was cut in a direction perpendicular to the longitudinal direction of each test number, and a disk having a surface (cross section) perpendicular to the longitudinal direction of the steel wire and a thickness of 0.5mm was collected. The disk was ground and polished from both sides using sandpaper to a thickness of 50 μm. Then, a sample having a diameter of 3mm was collected from the disk. The sample was immersed in a 10% perchloric acid-glacial acetic acid solution, and electropolishing was carried out to prepare a film sample having a thickness of 100 nm.

Using the film sample thus produced, the number density (in units/μm) of V-based precipitates having a maximum diameter of 2 to 10nm was obtained in the same manner as in example 1 ³ ). The number density of V-based precipitates obtained was set to "number density of V-based precipitates (number/. Mu.m) in Table 4 ³ ) "shown in column. "number density of V-series precipitate (mu m) ³ ) "in a column" - "means that the number density of V-based precipitates is 0 pieces/μm ³ . The number density of V-group precipitates was also measured for each test number of nitrided valve spring by the same method as that obtained in the steel wire. As a result, the number density of V-system precipitates in the core portion of the valve spring subjected to nitriding treatment for each test number is the same as the number density of V-system precipitates in the steel wire for the corresponding test number. The number density of V-group precipitates was also measured for each test number of valve springs that were not nitrided by the same method as that obtained in the steel wire. As a result, the number density of V-system precipitates of the valve spring which is not nitrided for each test number is the same as the number density of V-system precipitates of the steel wire for the corresponding test number.

[ Ca sulfide number proportion Rca measurement test ]

Test pieces were collected from cross sections of the central axes of steel wires containing each test number. The surface of the collected test piece corresponding to the cross section including the center axis of the steel wire was used as the observation surface. The observation surface was mirror polished. The observation field of view (each observation field of view: 100 μm. Times.100 μm) was observed at any 10 places from the surface R/2 position of the steel wire in the mirror-polished observation plane at a magnification of 1000 times using SEM.

Inclusions in each observation field are determined based on the contrast in each observation field. For each of the identified inclusions, oxide inclusions, sulfide inclusions, and Ca sulfide were identified using EDS. Specifically, based on the elemental analysis results of the inclusions obtained by EDS, the inclusion having an O content of 10.0% or more in mass% among the inclusions was identified as an "oxide inclusion". Among the inclusions, those having an S content of 10.0% or more and an O content of less than 10.0% by mass were identified as "sulfide-based inclusions". Among the sulfide-based inclusions thus determined, an inclusion having a Ca content of 10.0% or more, an S content of 10.0% or more, and an O content of less than 10.0% in mass% was determined as "Ca sulfide".

The inclusions to be identified are those having an equivalent circle diameter of 0.5 μm or more. The beam diameter of EDS for specifying inclusions was set to 0.2. Mu.m. Based on the total number of oxide inclusions and sulfide inclusions determined in the observation field of view of the 10 positions and the total number of Ca sulfides determined in the observation field of view of the 10 positions, the Ca sulfide number ratio Rca (%) is determined using the formula (1).

[ Vickers hardness measurement test ]

The hardness of the core of the nitrided valve spring of each test number was obtained by the vickers hardness measurement test. Specifically, a vickers hardness measurement test was performed on the basis of JIS Z2244 (2009) at any 3 of the positions of the cross section R/2 in the wire diameter direction of the valve spring subjected to nitriding treatment for each test number. The test force was set at 0.49N. The arithmetic average of the obtained vickers hardness at 3 positions was taken as the vickers hardness of the core of the nitriding valve spring of the test number.

Similarly, the hardness of the valve spring, which was not nitrided, was obtained for each test number by the vickers hardness measurement test. Specifically, a vickers hardness measurement test was performed according to JIS Z2244 (2009) at any 3 of the positions of the cross section R/2 in the wire diameter direction of the valve spring, which was not nitrided, of each test number. The test force was set at 0.49N. The arithmetic average of the obtained vickers hardness at 3 positions was used as the vickers hardness of the valve spring that was not nitrided for the test number.

[ fatigue test ]

The following fatigue tests were performed using valve springs (nitrided and non-nitrided) of each test number. In the fatigue test, a compression fatigue test was performed in which a load was repeatedly applied to a coil-shaped valve spring (nitrided or non-nitrided) in the central axis direction. As the tester, an electrohydraulic servo type fatigue tester (load capacity 500 kN) was used.

The test conditions were: the load is applied at a stress ratio of 0.2 and the frequency is 1-3 Hz. Repeating for 10 times ⁸ And secondly, the upper limit is implemented until the valve spring breaks. At up to 10 ⁸ When the secondary valve spring is not broken, the test is stopped, and the secondary valve spring is judged to be unbroken. Here, will 10 ⁸ The maximum value of the test stress not broken at the time was set to F _M F is to F _M Above and reach 10 ⁸ The minimum value of the test stress at which the fracture occurred next time before was set to F _B . Will F _M And F _B Is set to F _A Will (F) _B -F _M )/F _A F in the case of.ltoreq.0.10 _A Defined as the fatigue limit (MPa). On the other hand, in the case where the test result is all broken, i.e., F cannot be obtained _M In the case of (2), extrapolation from the relationship between fracture life and test stress corresponds to 10 ⁸ Test stress for sub-life, the test stress obtained is defined as the fatigue limit (MPa). Here, the The test stress corresponds to the surface stress amplitude at the fracture site. For each test number of valve springs, the fatigue limit (MPa) at high cycle was determined based on the above definition and evaluation test. Further, using the obtained fatigue limit and vickers hardness, the fatigue limit ratio (=fatigue limit/vickers hardness of the core) of the valve spring subjected to nitriding treatment, and the fatigue limit ratio (=fatigue limit/vickers hardness) of the valve spring not subjected to nitriding treatment were obtained.

[ test results ]

The test results are shown in Table 4. Referring to table 4, in test numbers 1 to 21, the chemical composition was appropriate, and the manufacturing process was also appropriate. Therefore, the martensite area ratio was 90.0% or more in the microstructure of each test-numbered steel wire. In addition, the number density of V-series precipitates with the maximum diameter of 2-10 nm is 5000-80000/mu m ³ . The Ca sulfide number ratio Rca is 0.20% or less. Therefore, the fatigue limit of the nitrided valve spring manufactured using the steel wire as the raw material is 1390MPa or more, and the fatigue limit ratio (=fatigue limit/vickers hardness of core) of the nitrided valve spring is 2.45 or more. The valve spring produced by using the steel wire and not subjected to nitriding treatment has a fatigue limit of 1340MPa or more and a fatigue limit ratio (=fatigue limit/vickers hardness) of 2.35 or more.

In test No. 23, the V content was too low. Therefore, the number density of V-based precipitates having a maximum diameter of 2 to 10nm is too small in the steel wire. As a result, the fatigue limit of the valve spring subjected to nitriding treatment was lower than 1390MPa, and the fatigue limit ratio was lower than 2.45. In addition, the valve spring without nitriding treatment has a fatigue limit of less than 1340MPa and a fatigue limit ratio of less than 2.35.

In test No. 24, the Ca content was too low. As a result, the valve spring subjected to nitriding treatment is highly cycled (10 ⁸ Secondary) is below 1390MPa and the fatigue limit ratio is below 2.45. In addition, the valve spring, which has not been nitrided, is subjected to a high cycle (10 ⁸ Secondary) fatigue underThe limit is less than 1340MPa and the fatigue limit ratio is less than 2.35.

In test No. 25, the Ca content was too high. Therefore, the Ca sulfide number ratio Rca is too high in the steel wire. As a result, the fatigue limit of the valve spring subjected to nitriding treatment was lower than 1390MPa, and the fatigue limit ratio was lower than 2.45. In addition, the valve spring without nitriding treatment has a fatigue limit of less than 1340MPa and a fatigue limit ratio of less than 2.35.

In test nos. 26 to 28, although the chemical compositions were appropriate, no heat treatment for forming V-system precipitates was performed. Therefore, the number density of V-based precipitates having a maximum diameter of 2 to 10nm is too small in the steel wire. As a result, the fatigue limit of the valve spring subjected to nitriding treatment was lower than 1390MPa, and the fatigue limit ratio was lower than 2.45. In addition, the valve spring without nitriding treatment has a fatigue limit of less than 1340MPa and a fatigue limit ratio of less than 2.35.

In test nos. 29 to 31, the heat treatment temperature in the V-system precipitate formation heat treatment was too low, although the chemical composition was appropriate. Therefore, the number density of V-based precipitates having a maximum diameter of 2 to 10nm is too small in the steel wire. As a result, the fatigue limit of the valve spring subjected to nitriding treatment was lower than 1390MPa, and the fatigue limit ratio was lower than 2.45. In addition, the valve spring without nitriding treatment has a fatigue limit of less than 1340MPa and a fatigue limit ratio of less than 2.35.

In test nos. 32 to 34, the heat treatment temperature in the V-system precipitate formation heat treatment was too high, although the chemical composition was appropriate. Therefore, in the steel wire, the V-based precipitates coarsen and the number density of the V-based precipitates having a maximum diameter of 2 to 10nm is too small. As a result, the fatigue limit of the valve spring subjected to nitriding treatment was lower than 1390MPa, and the fatigue limit ratio was lower than 2.45. In addition, the valve spring without nitriding treatment has a fatigue limit of less than 1340MPa and a fatigue limit ratio of less than 2.35.

In test nos. 35 and 36, the Ca content in the alloy iron added to the molten steel in the refining step exceeded 1.0%. Therefore, the Ca sulfide number ratio Rca is too high in the steel wire. As a result, the fatigue limit of the valve spring subjected to nitriding treatment was lower than 1390MPa, and the fatigue limit ratio was lower than 2.45. In addition, the valve spring without nitriding treatment has a fatigue limit of less than 1340MPa and a fatigue limit ratio of less than 2.35.

In test numbers 37 and 38, the time from the start of the refining step to the addition of the slag former was more than 4t/5 (0.80 t) (minutes). Therefore, the Ca sulfide number ratio Rca is too high in the steel wire. As a result, the fatigue limit of the valve spring subjected to nitriding treatment was lower than 1390MPa, and the fatigue limit ratio was lower than 2.45. In addition, the valve spring without nitriding treatment has a fatigue limit of less than 1340MPa and a fatigue limit ratio of less than 2.35.

In test No. 39, although the chemical composition was appropriate, fn defined by the formula (2) exceeded 38.9 in the V-system precipitate formation heat treatment. As a result, the number density of V-based precipitates having a maximum diameter of 2 to 10nm in the steel wire is too small. As a result, the fatigue limit of the valve spring subjected to nitriding treatment was lower than 1390MPa, and the fatigue limit ratio was lower than 2.45. In addition, the valve spring without nitriding treatment has a fatigue limit of less than 1340MPa and a fatigue limit ratio of less than 2.35.

In test No. 40, although the chemical composition was appropriate, fn defined by the formula (2) was lower than 29.5 in the V-system precipitate formation heat treatment. As a result, the number density of V-based precipitates having a maximum diameter of 2 to 10nm in the steel wire is too small. As a result, the fatigue limit of the valve spring subjected to nitriding treatment was lower than 1390MPa, and the fatigue limit ratio was lower than 2.45. In addition, the valve spring without nitriding treatment has a fatigue limit of less than 1340MPa and a fatigue limit ratio of less than 2.35.

The embodiments of the present invention have been described above. However, the above-described embodiments are merely examples for implementing the present invention. Therefore, the present invention is not limited to the above-described embodiments, and the above-described embodiments may be appropriately modified within a range not departing from the gist thereof.

Claims

1. A steel wire comprising, in mass%, the following chemical components:

C：0.50～0.80％，

si:1.20 to less than 2.50 percent,

Mn：0.25～1.00％，

p: the content of the organic acid is less than 0.020 percent,

s: the content of the organic acid is less than 0.020 percent,

Cr：0.40～1.90％，

V：0.05～0.60％，

n: the content of the organic light-emitting diode is less than 0.0100 percent,

the balance is composed of Fe and impurities,

2. The steel wire according to claim 1, wherein,

the chemical composition contains Ca:0.0001 to 0.0050 percent,

3. The steel wire according to claim 1, wherein,

mo: less than 0.50 percent,

Nb:0.050% or less,

W: less than 0.60 percent,

Ni: less than 0.500 percent,

Co:0.30% or less

B: less than 0.0050%.

4. The steel wire according to claim 2, wherein,

the chemical composition contains 1 or more than 2 elements selected from the following elements: mo: less than 0.50 percent,

Nb:0.050% or less,

W: less than 0.60 percent,

Ni: less than 0.500 percent,

Co:0.30% or less

B: less than 0.0050%.

5. The steel wire according to any one of claims 1 to 4, wherein,

the chemical composition contains 1 or more than 2 elements selected from the following elements: cu:0.050% or less,

Al:0.0050% or less

Ti:0.050% or less.